>> Hello and welcome to another
Fermilab at home lecture series.
I'm Tom Carter, your host for this evening and
I'd like to say good evening, good afternoon,
and even good morning to our audience that's in
just about every single time zone on the globe.
I'd like to say a bit about our
upcoming talks in this series.
On September 11th, Dr. Don Lincoln will speak
on Understanding God's Thoughts:
Einstein's Unfinished Dream.
Dr. Lincoln's going to explain the current
status of Einstein's timeless quest
and give the audience a sense for
the prospects of completing his work.
On October 16th, Dr. Jennifer Raaf will speak
on How Particle Physics Might Save Your Life.
And she's going to talk on how particle
physics scientists were involved
in building a simplified mechanical ventilator
as part of the COVID relief effort.
A bit about the mechanics of this
night's talk, a closed caption recording
of this talk will be available on
our website shortly after we're done.
Your audio feed will be muted throughout
the talk, but you can ask questions
by typing in the Q&A link on your screen.
If you're using a tablet you might have to
touch your screen for that Q&A link to show up.
And the speaker will answer the
questions at the end of the talk.
So you can ask a question anytime you
like, we'll get to them at the end.
And now tonight's speaker, Dr. Dan Hooper.
Dr. Hooper's Ph.D. is from the University of
Wisconsin and he is currently a Senior Scientist
and the Head of the Theoretical
Astrophysics Group here at Fermilab,
along with being an Associate
Professor at the University of Chicago.
Prior to this, he was a David
Schramm Fellow at Fermilab
and a postdoc at the University of Oxford.
He's the author of three books for
the general public on cosmology,
the third of which just came out
recently on Princeton University Press.
He's created a fascinating course, a science
course titled "What Einstein Got Wrong" as part
of the greats -- Great Courses
series and that's available on DVD.
And to top it all off, Dan is the bass
player in the soul band The Congregation.
So please welcome Dan Hooper.
>> Hi, everyone.
I'm very excited to be giving this talk.
I'm a big fan of the Fermilab public lecture
series and even though I would strongly prefer
to be doing this in person, I am just
thrilled to see so many people register
for this more virtual form
of these sorts of lectures.
So let me begin with some kind
of sweeping or grand remarks.
I think it's remarkable just to think
about the fact that throughout the entirety
of human history people in all times and in
all cultures have looked up at the night sky
and wondered about their
universe and how it came to be.
In this respect, we have a lot in
common with our ancient ancestors,
but in one important way
we're different, we're unique.
We're unique in the sense that we happen to
be lucky enough to be living at the one time
in history where we can look up at the
night sky and honestly say that we more
or less understand what it is we're looking at.
Take this image, for example.
This is something -- an image taken
by the Hubble Space Telescope is part
of what is known as the Deep Field program.
Most of the blotches of light in this image
are not stars as you might've guessed,
but they're actually entire galaxies similar
in size and shape to our own Milky Way.
But because it takes time for light to travel
across space this picture doesn't show us what
these galaxies are like now, but what they were
like over 13 billion years ago, only
400 million years after the Big Bang.
Our universe was a very different
place than it is today.
It was smaller, it was more
compact and it was hotter.
And we can honestly say that
we understand pretty well how
and why our universe has
transitioned from this early state
into the universe we find
ourselves living in today.
A little over a century ago, science
didn't have anything to say at all
about our universe's distant past, and
certainly nothing to say about its origin.
The laws of physics as we understood them before
the 20th century simply didn't have the --
well give us the tools that we
needed to conceptualize how something
like space could transform or begin.
But this all changed with the invention
or introduction of the general theory
of relativity by Albert Einstein in 1915.
Prior to Einstein, physicists thought
of space as an unchanging backdrop,
something that was static through
which objects could potentially move.
But Einstein showed us that
this is only the beginning
of what space and time is really all about.
He showed us that space could do things.
Space could curve, it could warp, it could
change, it could contract, it could expand.
In fact, the equations of Einstein's theory
could be used to show that the one thing
that the space that constitutes our
universe could not do is stay the same.
It really had to be contracting
or expanding with time.
And in 1929, the astronomer Edwin
Hubble with his collaborators showed
for the first time observationally
that our universe is in fact expanding.
The volume of any given piece of our universe
is getting larger as time progresses.
So if you're like most people you've heard
before that the universe is expanding.
This is part of a popular culture at this
point, but also if you're like most people,
you probably don't have a very good
idea what this really means exactly.
So let me try to shed some
light on this subject.
What Hubble actually observed with
his telescope in 1929 is that all
of the galaxies he could image
seem to be moving away from us.
Furthermore, the farther away from us a given
galaxy is the faster it seemed to be receding.
We now understand that what this -- what he
was really observing is the fact that space,
the amount of space between any two points in
our universe is getting larger as time advances.
The volume of space itself
is growing as time goes on.
Now, I've given a lot of public lectures
on this subject and related subjects,
and I've done it enough in-person to know that
around this time in the talk a large fraction
of my audience comes up with the same question.
And I met a lot of you are
wondering the same thing right now.
If I were in the room and with -- in-person with
you I would ask for a show of hands of how many
of you right now are asking yourself the
question, what is the space expanding into?
It sounds like a pretty good question,
but it doesn't have a very good answer.
The reason it doesn't have a very good
answer is because if space were expanding
into something else I would
call that something else space.
And when I say space is expanding I don't
mean that part of space is expanding.
I mean that all of space is
simultaneously expanding in unison.
So when cosmologists say that the universe is
expanding they don't mean that part of space
or part of the things in space are moving
into some previously unoccupied part
of the universe rather they
mean that the entirety --
the volume of the entire universe
is getting larger as time goes on.
From the fact that space is expanding,
we can begin to infer some things
about our universe's distant past.
In particular, since is expanding that means the
volume of any given piece of space was smaller
in the past and in order to fit all
of the matter and energy that we have
in our universe today in that smaller
volume, we can deduce that the universe
in the past must've been much
more dense and much more hot.
This is the basis of what
we call the Big Bang theory.
The idea that over billions of years
our universe has expanded and evolved
from a hot dense state that we call the Big Bang
into the universe we find
ourselves occupying today.
Before moving on I want to comment on one
common misconception about the Big Bang.
When people think of the Big Bang they can
be tempted to imagine that it was an event
that took place at some particular location in
our universe, like a cosmic explosion that grew
out of a location into the rest of the
universe, but that's not what we mean.
When we talk about -- when cosmologists talk
about the Big Bang they don't mean something
that happened somewhere specifically in space.
They mean something that was a state that the
entire universe was in 13.8 billion years ago.
So you know, the Big Bang
that happened, you know,
here or anywhere else it happened everywhere
at the same time, including the place
in our universe we occupy right now.
All right.
So let's take a little bit of a different
pivot here and let's talk about the timeline
of our universe's history, the main events
that have unfolded since the Big Bang.
So here I show a timeline, a cosmic timeline,
and a few -- the key events are shown on it.
In particular, you can see where
the first stars began to form
about 200 million years after the Big Bang.
These stars were a lot larger
and shorter-lived than most
of the stars we have in our universe today.
And only now are astronomers about
to introduce the kind of technology
that we think will produce the first
images of this first generation of stars.
About 9 billion years after
that, our own star, the Sun,
and the planets that make up
our solar system all formed.
And then about 4.5 billion
years later life in the form
of human beings emerged on our planet surface.
Notice that on this chart all of human
history from Homo sapiens and Homo habilis
and prehistory all the way forward
occupies less than a fraction of a pixel.
From this perspective, human
-- the role of human beings
in the universe might seem
inconsequential even though to all
of us I'm sure human history has
played an outsized role in our view
of important things in our universe's history.
So the timeline we're looking at right
now there's nothing wrong with it,
all the things I've drawn on it are correct.
But as far as a cosmologist might be concerned,
this is a pretty dull way
of showing cosmic history.
It's dull because it doesn't
emphasize the most exciting parts
of our universe's evolution or unfolding.
Instead, we like to show timelines that
are a little bit more like this one.
Instead of a linear timescale here,
I'm showing a logarithmic timescale.
Each tick mark shows a factor
of 10 in time passing.
This allows us to look much, much closer to
the Big Bang itself and farther back in time
where many of our most exciting
events took place.
In particular, you can see on this version of
the timeline the transition that took place
about 380,000 years after the Big Bang,
when the first atoms began to form.
These atoms began to form at this
time because of the temperature
that the universe was maintained at this moment
in cosmic history, which is about 3,000 degrees.
There's a background of radiation that fills
our universe today, but it's cold today.
It's only about 2.7 degrees above absolute zero.
But 380,000 years after the Big Bang, the entire
universe was filled with a plasma of photons
and electrons and other charged particles
maintained to the temperature of 3,000 degrees
or about the temperature of
the surface of a red star.
It was around this temperature that atoms began
to form because I like to think of 3,000 degrees
as the melting point of many kinds of atoms.
What I mean by this is if I took, oh, some atoms
and I heated them up to a temperature greater
than 3,000 degrees they would
begin to fall apart.
Their electrons would break off.
So instead of a gas of electrically neutral
atoms, you'd find yourself with a plasma
of photons, electrons, and nuclei all
interacting in a frequent complex way.
For a plasma, it turns out light cannot
generally travel through such a medium.
The plasma is opaque to light.
Shining a beam of light through the whole
universe would be about as effective
as shining a flashlight directly
into the Earth today,
just will not penetrate that sort of medium.
But as the universe expanded and cooled
eventually those electrons began to bind
to those nuclei forming electrically neutral
atoms or complete atoms for the first time.
And as this happened, our universe
got more and more transparent.
So as our universe evolve from an opaque state
into a transparent state suddenly an
enormous amount of light was deposited
into every corner of our universe.
This light has been traveling
through the universe ever since
and it, in fact, fills our universe.
What I'm referring to is a cosmic microwave
background, the light that was generated
in the formation of the first atoms that
we can measure in great detail today.
What I'm showing you here is an image of that
very light the cause of microwave background.
The hottest and coldest parts of
this light tells us where the least
and greatest density regions of our universe
were 380,000 years after the Big Bang.
This not only tells us what our universe
was like in that primordial state,
but what forms of matter
and energy occupied space
and how fast our universe has
expanded and evolved ever since.
This data set is truly a
treasure trove to cosmologists
and it gives us a great deal confidence that
we understand our universe and its history
from a few hundred thousand years
after the Big Bang up to the present.
All right.
So let's go back to our timeline, but now
let's go back to an even earlier time.
Let's go back to the first minutes
and seconds after the Big Bang
at which point the temperature of the
universe was around a billion degrees.
That's hotter than the core of
the Sun, and it's a temperature
at which nuclear fusion can
proceed extremely efficient.
So you can think of the universe at this time
as being one giant nuclear fusion reactor.
And it was around this time that the
first atomic nuclei were being formed.
Prior to this point, there were protons and
neutrons in the universe, but no nuclei.
The protons and neutrons were
not able to fuse into one another
until these first minutes and seconds.
And through fusion, they form things
like deuterium, tritium, helium,
and even a smattering of heavier
things like lithium and beryllium.
Using Einstein's theory of general relativity
combined with facts they accumulated
about nuclear physics over the course of
the 20th century, cosmologists were able
to predict how much of these different
kinds of elements should have been formed
in the universe's first seconds and minutes.
And then we went and compared those predictions
to actual measurements of the abundances
of things like helium, hydrogen,
lithium, and beryllium in our universe.
And lo and behold, we found a really
good match, a remarkably good match.
This gives us a great deal of
confidence that our standard picture
of the early universe's evolution describes our
universe quite well, at least from a few seconds
after the Big Bang up to the present.
Going back to our timeline we can go back even
further to roughly a millionth of a second
after the Big Bang, at which
point the temperature
of the universe was about 10 trillion degrees.
And it was around this time that we think
the first protons and neutrons were forming.
Protons and neutrons are made of
smaller particles called quarks
and held together by things we call gluons.
And at temperatures greater than
about 10 trillion degrees those quarks
and gluons will simply fall apart.
The protons and neutrons that we know
and love simply cannot remain intact
under such incredible temperatures.
But around this millionth of a
second point, those quarks begin
to bind together transitioning from an
era of free quarks and gluons into one
in which protons and neutrons existed instead.
So one -- before in this talk when I talked
about the formation of the first nuclei,
the formation of the first
atoms I gave you good reasons,
good empirical reasons why we're confident these
things happen the way the theory says they do.
We have the cosmic microwave background,
we have the light nuclear elements
that we've actually measured the abundances
that we have, predictions that we compare
against observations to test
our theories robustly.
When it comes to the formation of the first
protons and neutrons a millionth of a second
after the Big Bang we have nothing like this.
We do not have any telescopes that can
observe this era of cosmic history and nothing
about the protons or neutrons that exist in our
universe leave us confident that we understand
when or how they precisely formed.
But just because we can't observe this era
doesn't mean we can't try to learn about it.
But instead of trying to observe this era, we do
it -- we go about this in a very different way.
We try to build experiments in the laboratory
that actively recreate the
conditions of the very early universe.
These machines or these experiments
are called particle accelerators
and they are our most powerful way about --
for learning about our universe's first second.
This is an image of the world's most powerful
particle accelerator, the Large Hadron Collider.
So essentially a 17-mile tunnel which goes
underground beneath the City of Geneva,
Switzerland and into nearby neighboring France.
Around that tunnel powerful magnets accelerate
protons to speeds just below the speed of light.
Design operation it reaches a speed of
about 99.999997% of the speed of light.
And then those beams of protons are
directed head-on into one another inside
of these giant gymnasium-sized
particle detectors.
These collisions take place around
600 million times every second
and sophisticated electronics
study all the sorts of particles
that come out of these collisions.
And by studying a very, very large
number of these collisions at very, very,
very high energies or speeds we can learn
an enormous amount about the laws of physics
as they pertained in that first trillionth
of a second or so after the Big Bang.
This chart shows all the forms of matter
and energy that we'd ever observed
or discovered using these
particles accelerators.
This includes six types of quarks, the up and
down quarks that make up protons and neutrons,
but also more exotic forms of quarks, the
charm, strange, top, and bottom quarks.
The top quark was discovered here at
Fermilab back in 1995, for example.
And also six kinds of leptons including the
well-known electron its heavier unstable
cousins, the muon and tau, and three very
feeble interacting particles called neutrinos.
And then there were the force-carrying
particles like the photon and gluon
that I've already mentioned and
also things like the W and Z boson.
And then there's the most recent addition,
the Higgs boson that was discovered
at the Large Hadron Collider back in 2012.
Most of these particles are
extremely rare in our universe today.
There are not W bosons and top quarks
flying around in any serious quantities.
But a trillionth of a second after
the Big Bang, the entire universe,
every last corner of our universe was filled
with a dense soup containing
all of these forms of matter.
Electrons at that point were
not substantially more common
than top quarks or Z bosons or Higgs bosons.
These particles all played instrumental or key
pivotal roles in how our universe evolved during
that first trillionth of a second.
All right.
So up to this point in the lecture, you might
be under the impression that what I'm trying
to tell you is that we understand
the first trillionth of a second,
the second of our universe quite well.
I would love if that were true,
but it's just not the case.
Sure we have a great theory,
a spectacular theory.
When we combine what we know about Einstein's
theory of relativity with what we've learned
about quantum or particle physics
from particle accelerators,
we have a theory that predicts a great
deal of what we observe very, very well.
We can explain the detail patterns of light that
we observe in the cosmic microwave background,
we can observe why the nuclear
elements come in the quantities
that we find them to be in our universe.
We can also explain things like why and
how stars formed, or galaxies formed,
the clusters of galaxies form, and
why those objects look the way they do
in our universe today.
All of these things are explained well
by our standard cosmological theory.
But when we look into the first second or so
after the Big Bang we don't have this kind
of information reliant to find out if the
theory we're using is complete or even correct.
Furthermore, in recent years and decades
cosmologists have stumbled upon a number
of puzzles that they haven't
been able to resolve
with their standard theory
of the early universe.
These puzzles might just be loose ends which
will be tied up or resolved in the years
or decades ahead with greater observations and
measurements or maybe even greater theorizing.
Or maybe they're telling us that we're
thinking about the early universe
in a substantially incorrect
way or incomplete way.
Let me describe these four puzzles and
we'll come back to the bigger implications.
The first of the four puzzles
I'm talking about has to do
with the simple existence
of atoms in our universe.
Everything we know about the laws of
physics say that for every kind of matter,
there must exist a copy that we call
antimatter with opposite quantum properties.
So for example, the electron is
a negatively charged particle
and its antimatter counterpart is something we
call a positron, which is a lot like an electron
but with a positive electric charge.
These two particles, the electron,
and the positron are the same mass,
basically all the same features, except for
things like electric charge which are reversed.
Similarly, quarks have antiquarks and
neutrinos have antineutrinos so on and so forth.
Matter and antimatter are perfect copies of
each other in almost every way we can measure.
When cosmologists ponder this they
run into a problem quite quickly.
From what we know about matter and antimatter
we should have expected the early universe
to have contained a perfectly
equal amount of matter
and antimatter shortly after the Big Bang.
And then as the universe
expanded and cooled that matter
and antimatter should have destroyed each other.
After all, we know from the laws of physics
as we measure them in particle accelerators
that you can't create or
destroy matter without creating
or destroying an equal amount of antimatter.
The fates of these two substances
are closely intertwined.
So if that theory was right, if everything
we knew was the end of the story,
then our early universe should've
expanded all the matter
and antimatter should've been destroyed
and there should be no remaining
matter in our universe today.
And that means no atoms, no stars, no planets,
no molecules, no life and certainly not us.
So this is obviously not the right answer
and we don't know what the resolution is,
but it's clear this is telling us things were
going on in the first fraction of a second
after the Big Bang that we
do not currently understand.
The second puzzle also has to do with matter,
but not the kind of matter
that's made up of atoms.
Instead, we're talking about
a different kind of matter,
something we don't really know what it is yet,
but something that doesn't appreciably radiate
reflect or absorb light, something that we call
for the lack of a better name, dark matter.
These images of the Galaxy Andromeda
or M31 as it's sometimes called,
this happens to be the Milky
Way's nearest neighbor.
And when you point telescopes at this you can
measure where things like the stars and gas
and dust and planets and
objects in the system are.
We can get a pretty good inventory of the matter
in the form of atoms inside of this galaxy.
From that information, we can deduce how fast
we think stars should be in orbits around it.
Basically, by looking at where all the
atoms are and applying the laws of gravity
as we understand them, we should be able
to deduce how fast different stars should
zip around a galaxy like Andromeda.
And when we do that we get a curve like this,
this is called a galactic rotation curve
and it basically is saying
that as you go farther away
from the galactic center these stars should
be moving in slower and slower orbits.
This is basically for the same reason
that we find Pluto moving more slowly
around the Sun than the Earth does.
In the '90s and '70s and '80s
however, astronomers actually went out
and measured these sorts of rotation curves,
people like Vera Rubin and Kent Ford and others
and they didn't find what people had expected.
Instead of the predicted curve,
they found rotation curves
which were flatter and faster
than we're expected.
This tells us two things.
First of all, it tells us that most of the
matter in these galaxies does not consist
of atoms, but of something else, something
like I said before we call dark matter.
Secondly, it tells us that the
dark matter is not distributed
in the same way as atomic matter is.
Instead of being in a relatively
compact disc, the dark matter exists
in a much more extended halo
approximately spherical extending
out to much greater distances
from the galactic center
than the visible components
of these galaxies do.
By the mid-1980s it had become clear to most
astrophysicists and most particle physicists
as well that the dark matter probably
doesn't consist of any of the forms
of matter we've ever seen, but of some sort of
new exotic elementary particle or maybe more
than one kind of particle, we're not sure which.
If that's true, then you can begin to calculate
how such a population of particles should evolve
over the course of our universe's history.
The image you're looking at now is
a result of a computer simulation
that takes an approximately uniform distribution
of dark matter and lets the force of gravity act
on that dark matter as the universe expands.
So we can see as time advances in this in this
simulation, the gravity tends to hold the clumps
of dark matter into each other
forming denser and denser objects
and eventually you reach the point wherein the
bottom right frame you can see there are very,
very dense objects filled by giant
voids in all their locations.
If you compare this image to the observed
distribution of galaxies and galaxy clusters
in our universe you find remarkable agreement.
What this tells us is that the dark matter
form the scaffolding of large-scale structure
of our universe over cosmic history.
In other words, galaxies and
clusters of galaxies formed
because the dark matter's gravity hold
things like atoms together to form the stars
and planets and dust and gas
that we can see with our eyes.
Dark matter played a pivotal role in the
formation of our universe as we know it.
If you asked me 10 years ago what I thought
the dark matter was likely to consist of,
I would have given you an impassioned
and confident-sounding speech
about things we call WIMPs, weakly
interacting massive particles.
And I would've told you that we understand
how these things would if they exist,
how they would've been formed
in the early universe
and how many should have
survived those conditions.
And lo and behold the abundances we were --
expected these particles to emerge from the
Big Bang with approximately match the amount
of dark matter we have in our universe.
That gave me -- us a lot of confidence that
dark matter probably did consist of WIMPs.
Further exciting the story is
that if dark matter did consist
of WIMPs we thought we knew how
to go out and build experiments
that would detect these particles for the first
time allowing us to measure their properties
and understand not only the
dark matter himself --
itself, but also understand
how it may have been formed
in the first fraction of a
second after the Big Bang.
So we carried out these experiments,
we did it very, very well.
In this image you see an underground laboratory
in Italy known as Gran Sasso Laboratory
where there are several dark matter
detectors, including the xenon detector
or the DarkSide Experiment, DAMA, CRESST,
all of these are dark matter detectors
buried deep underground in this laboratory.
These experiments performed as well
or better than we could've hoped.
They've improved in sensitivity by more than
a factor of 10,000 over the last decade.
It's truly is experimental
physics success story and
yet they never found any dark matter
particles or at least not anything that many
of us have become convinced are
authentic signals of dark matter.
This is making us rethink not only
what dark matter might be made of,
but how it might've been
formed in the early universe.
Dark matter-s no-show has given us a lot to
think about not only in terms of what sort
of things exist in our universe,
but how they came to exist
in our universe in the first place.
The third puzzle I'll mention has to do
with how fast our universe has
been expanding over its history.
If you take Einstein's theory,
general relativity
and you make the reasonable-sounding assumption
that most of the energy in our universe is
in the form of matter, then you find that
space should expand in one of three ways.
Either it should expand for a while reach
a maximum size and then begin to contract
like in the lower curve shown
in this image -- at this image.
Or it should expand forever without
limit, like in the top curve.
Or if you're just the right amount of
matter kind of the Goldilocks scenario,
you could have a scenario where
the universe expands for a while,
and then it kind of plateaus to a maxim size.
Those are kind of the three options, our
multiple-choice question had A, B or 3 --
A, B or C. But in the 1990s we built
telescopes that for the first time were able
to actually find out which of these cases
most accurately described our universe
and when those measurements were conducted they
got an answer which was D, none of the above.
Instead of a universe's -- universe
whose expansion rate was slowing
down with cosmic time, these astronomers learned
that it has been accelerating
over the past few billion years.
This was very surprising and it made
us rethink a lot of our assumptions
about our universe and the
energy that it contains.
In particular, the only way we really
have to understand this behavior,
the fact that our universe is -- its
expansion rate has been accelerating is deposit
that the very vacuum of space contains
energy, something we call dark energy.
Basic idea here is that if I take a cubic
meter of space at any time or any place
in our universe's history and I take all the
forms of matter out of it, all the photons,
all the neutrinos, all the atoms,
all the dark matter, everything,
it would still contain a
certain fixed density of energy.
As the universe expands other forms of matter
and energy get diluted by the expansion
of space, but not this dark energy.
And that means that as history has played
out, as cosmic history has played out,
dark energy has played an increasingly
important role slowly coming
to dominate the total energy
density of our universe
and driving its expansion rate to accelerate.
And fourth but not least that
last but not least is the puzzle
that cosmologists refer to as cosmic inflation.
If you took the Big Bang theory as it was
usually described in the 1970s or 1960s,
you could not explain why our universe
seem to be so uniform and why the geometry
of our universe was what's known as
Euclidean, the kind of geometry that you learn
about in high school math classes.
There's no reason either of those things
should've been true, according to what we knew
about the Big Bang theory and
cosmology in general, but it was.
We measured it, and as we measured it in
subsequent years we found it to be very, very,
very accurately true and this
poses a big problem for the
at least old version of the Big Bang theory.
In the 1980s a bunch of physicists and
cosmologists proposed and developed ideas
in which shortly after the Big Bang
space expanded in a hyper-fast burst,
increasing in volume by a factor of 10 to the
75 or more in only 10 of the minus 32 seconds.
We call this burst of expansion a cosmic
inflation and it could explain these features
that otherwise had been very
confusing to us about the Big Bang.
We still don't know for sure that inflation
took place, but the theories worked
out in the 1980s made a number of very precise
predictions or very specific predictions
for what we'd had observed in
the cosmic microwave background.
And as time has played out
and those measurements were conducted
we found those predictions to be true.
And this has convinced most
cosmologists that something
like inflation probably took
place shortly after the Big Bang.
But we still don't know exactly
why inflation occurred,
what drove it to happen or
what caused it to end.
One of the things I find most intriguing
about inflation is that at least in most
of our theories of inflation it never ends.
A piece -- a patch of space begins to expand and
in only little bit of that expanding space do --
does inflation end causing
a hot sort of dense universe
like the one ours was once in to emerge.
The rest of it continues to inflate and
as this process goes on more and more
of these pocket universes kind of
fall out of the process of inflation.
So from this perspective, you would expect
a universe underwent inflation at least
for a brief period of time
would have produced an infinite
or nearly infinite number of universes.
From this perspective, it seems that
modern cosmology is giving us an argument
for why we should expect that
our universe is only one of many,
a small part of a greater cosmic multiverse.
So let me try to put all this in
perspective with a little bit of a lesson I
like to refer to from the history of science.
When I'm talking about the state of cosmology
and the state of the Big Bang theory today
to my colleagues I try to get them
to think about whether we're going
to resolve these puzzles that I've been talking
about for the last 20 minutes
or so with small changes.
Or incremental changes to the Big Bang
theory as we know it, or are we going to have
to replace subsequent parts of the Big
Bang theory with something truly different,
something we're not currently thinking about.
In other words, are we going to tie
up some loose end or are we going
to replace the existing cosmological
paradigm with something very different?
And to get them to think about this I like to
ask them the following question, I like to ask,
what do you think it would've been
like to have been a physicist in 1904?
The reason I pick 1904 in the story is because
it at least in my reading of the history
of science, 1904 was the year in which
physicists felt the most confident
that they truly understood their universe.
In 1904 for 200 years or so the theories laid
out by Isaac Newton had just continued to work.
Newtonian physics as a paradigm had explained
gravity and the orbits of planets and any number
of dynamical systems and it -- when it was
applied to later ideas like electricity
and magnetism and heat those
same principles were found
to be successful in any variety of application.
Physicists not only thought that classical --
Newtonian physics had been successful for
the past 200 years, had every confidence
that it would be successful for the
next 200 if not 2,000 additional years.
They thought they had really discovered
the key principles to our universe.
Of course, in 1904, they also knew there
were a few loose ends that had proven
at least resistant to explanation
in the Newtonian paradigm.
Let me tell you about a few of those problems.
The first of their problems had
to do with the nature of light.
By 1904 physicists knew that light was an
electromagnetic wave, but it behaved differently
than other waves they knew about.
If I told you that a water wave was moving
through the ocean at 50 miles an hour,
we would all agree that if you got in a boat and
moved at 50 miles an hour in the same direction
as a wave you would find the wave to be moving
at rest or not moving at all relative to you.
In other words, the speed of a wave
depended on your frame of reference.
But when light had been measured
in a variety of different speeds --
frames of reference they
always measured the same speed.
The speed of light seem to be a
universal quantity and no one knew how
to explain that, at least not in 1904.
The second problem or puzzle had to do
with the orbit of the planet Mercury.
Newtonian physics predicted like all the
planets, Mercury should be moving on around
in the Sun on an elliptical
orbit and the orientation
of that ellipse should change
slightly from year to year.
So technically speaking, we say it's a
perihelion should be processing from year
to year, its orientation should change.
And you could calculate the rates of that
procession or at least what it should have been.
But throughout the course of the 19th
century had become clear that Mercury's rate
of procession is a little bit different than
what was predicted by Newtonian physics.
Some astronomers posited that maybe
there was an additional planet out there
that had not been discovered
yet, a planet they called Vulcan.
That might have been gently tagging
on Mercury perturbing its orbit.
But when they looked for this
planet they didn't have any success.
So I think it's fair to say that in
1904, no one had any really good ideas
for what was going wrong
with the orbit of Mercury.
The third puzzle, which I think
is the most remarkable one
and how outstanding it should have been, how
much it should have been on everybody's mind was
that 1904 no physicists or anyone else had
any idea where the Sun got its energy from.
The Sun has given off an enormous amount of
energy in the form of starlight over billions
of years, and there just isn't any
kind of physics that was known in 1904
that could produce anything
close to this much energy.
Even if the entire Sun had been made of some
combustible material like coal or gasoline
or TNT or something, it would have
run out of energy long, long ago.
In 1904, there was simply no answer to
the question of why does the Sun shine?
And then last but not least was
the simple fact that physicists
in 1904 couldn't build a
workable model of the atom.
By 1904, physicists had at least begun to
understand some things about the structure
of atoms, but if you picture
something like this a planetary model
where there's a big dense nucleus
orbited by planet-like electrons,
classical physics tells you
those electrons should crash
into the nucleus in any fraction of a second.
In other words, atoms should
be entirely unstable.
All of the atoms throughout our universe
should simply collapse in a picosecond
or less if the laws of classical
physics were correct.
So in 1904, I think if you had a conversation
with a group of well-informed physicists,
they would probably tell you,
"Sure, these are problems.
But Newtonian physics has worked for so long,
if we just think about this harder we measure
some things better, we make some more progress
on these fronts, we'll find out how to turn
Newtonian physics against these problems
and provide real tangible solutions."
So that's how it's always
been for hundreds of years,
they were confident this would
happen again and they were wrong.
In 1905, Albert Einstein, a very young Albert
Einstein working in the Swiss patent office,
proposed solutions to these problems
by introducing the theory of relativity
and the beginnings of what would
become known as quantum physics.
These ideas were not tying up of loose ends,
they were tearing down the basic Newtonian
concepts of matter, energy and space and time
and replacing them with entirely
new paradigms of quantum physics
and Einstein's vision of
relativistic space and time.
So when I think about the state of
cosmology, I like to ponder the possibility
that 2020 might be the 1904 of cosmology.
It might be that where these loose
ends we've been struggling with for
so long are telling us something, not
just incremental and not just the tying
up of loose ends, but instead tell us that
we've been thinking about the early universe
in substantially incomplete or incorrect ways.
And maybe they're giving us clues
that will get us to some sort
of greater, more overarching theory.
Of course, I might be wrong, it may be that we
discovered what dark matter is, we understand --
we learn how inflation works, we understand
why matter was victorious over antimatter,
and we understand the nature
of dark energy in the years
and decades ahead leaving the
basic Big Bang picture intact.
But I kind of hope not because
it would be a lot more exciting
to see something more revolutionary play out
in the timescale of my career in physics.
So I'd like to thank the
organizers of this lecture series
for the opportunity to talk with you tonight.
I'm happy to take some questions in a little
bit, but first I'm going to do just a couple
of minutes of shameless self-promotion.
If you've enjoyed this talk and want
to hear more about the early universe
and are puzzles we're facing in modern
cosmology, I have this new book out called
"At the Edge of Time: Exploring the
Mysteries of Our Universe's First Seconds."
You can find that on Amazon or
wherever else you buy books.
People seem to like it so I guess I
recommend it from that perspective.
And also if you're more of an audio sort
of person than a book sort of person,
I've recently launched my new podcast with
Shalma Wegsman called "Why This Universe?"
And every Monday we put out a 20 or 30
minutes segment on some part of physics
that we think is awfully cool and
people seem to be having fun with that.
So check that out if you're
into that sort of thing.
And at this point, I'll turn
it over to the audience
and enthusiastically take
some of their questions.
Thanks again.
>> Okay. Dan, we've got a large
list of really great questions.
>> Awesome.
>> Okay. So I'm going to start with the big one.
My guess is, is you always get -- you
know, what was there before the Big Bang?
And I'm going to combine that with another
question is, what made the quarks and muons
that exist after the trillionth
of a second after the Big Bang,
that is where did these bits
of matter get formed from?
>> So I'm going to answer those
questions in kind of reversed order
because the second one is a
lot easier than the first one.
So the thing is, energy in
any form can be converted
in principle to energy in any other form.
So whatever you had prior to a trillionth
of a second, whatever forms of matter
and energy existed to that point they
can convert into things like top quarks
and muons and taus and Higgs bosons.
In the same way that in the Large Hadron
Collider we collide protons together
and we make all those things.
These are all the forms of energy that
exist and in fact, a trillionth of a second
after the Big Bang if I took a quark and I
just watch the energy, I trace that energy
as it traveled throughout the universe.
What you'd find is in a tiny, tiny, tiny
fraction of a trillionth of a second it would be
in some form or some other particle and then
another and then another and then another.
Energy did not take this form of a single
kind of particle for any length of time,
everything was fleeting, everything
was in flux, everything was volatile.
Now as far as what happened
before the Big Bang, this is --
I mean the honest answer is we don't know and we
don't even know if it's a well-formed question.
If you take what Stephen Hawking and Roger
Penrose and others worked out back in the '60s
and '70s, it would seem that there should
be no such thing as before the Big Bang.
There really is a beginning to time at what
we call the Big Bang and if there was no time
and no space prior to that there
was certainly nothing there
to talk about, nothing to cause the Big Bang.
The Big Bang or the existence of
the universe in this sense it has
to be a brute fact without cause.
The introduction or addition of cosmic inflation
to the Big Bang theory might
change that picture a bit.
After all, it's not obvious that inflation
ever had a beginning it could have --
it could be just going on forever.
And maybe you can talk about our universe
being created when it kind of popped
out of the inflating cosmos that
preceded it, but it's not obvious to me
that inflation ever had to have a start,
it may just be an eternally
occurring sequence of expanding space.
But I'll go back to the original part
of my answer, which is I think it's fair
to say we don't really know
the answer to this for sure.
>> Okay. This is maybe a
clarification question, Dan.
Somebody asked is during this whole time that
you are describing the universe was expanding
as time is ticking and if so how is
time passing and the universe expanding,
how are those two things related?
>> So as far as we can tell universe
expanding at a smooth and steady rate,
that means that if I take any two points
in space they are getting a little farther
apart from each other as time goes on.
Now we don't notice this in our day-to-day
lives because things are so close together here
and that means it would expand
very, very slowly.
We really only notice the expansion of space
when we look out at galaxies and clusters
of galaxies beyond the confines of Milky Way.
So it is true that we -- the space expanding
everywhere but not in a way that we perceive
in part because the force of gravity
tends to hold things like the galaxy
and the solar system together resisting
being pulled apart by the expansion of space.
And as far as how this relates to time, I mean
we use time as a tool to measure distances
between events and any number of ways.
I think the expansion of space we think about
-- we think of time and the context expansion
of space the same way that we think about time
in terms of orbits of the Earth around the Sun
or any other kinds of measure of
time that you might have in mind.
>> Okay. Here's a sort of
an experimental question.
How do cosmologists measure the relative amounts
of elements that exist in today's universe?
Is it in excess that was predicted
by -- from primordial nucleogenesis?
How do we actually -- do that measurement?
>> So there are a lot of different ways I think,
you know, one could give a whole hour talk
on ways that people go by doing it.
But like one of the most common though is that
we know different kinds of atoms emit light
at different wavelengths or frequencies.
So if we find a big gas of clouds somewhere
maybe a really old primordial gas cloud,
we point a telescope at it, we measure
those specific wavelengths and frequencies,
we know that that's coming from say
hydrogen or helium or lithium or beryllium.
And we do this in enough different clouds
and enough different times and locations
in the universe and we can
get a pretty good measurement
in some cases extremely accurate measurement of
how much of these different elements are present
in those primordial collections of gas.
>> Okay. Let me ask this.
Is there anti-dark matter?
>> That's a great question.
Again, I'll go to my standard answer
to these sorts of hard questions
that we don't really know because
we don't know what dark matter is.
But I'm confident that dark matter
either has anti-dark matter counterpart
or dark matter is itself anti-dark matter.
What I mean by that is if I take for example
a photon, which doesn't have anything
like electric charge or anything like that
and I take an anti-photon,
they're actually the same thing.
The photon and anti-photon because they don't --
it's like the negative number of zero is
still zero, the anti-photon is just a photon.
And it's possible the dark matter
could be identical to the anti-dark
and dark matter particle and since there
really wouldn't be anti-dark matter.
On the other hand, it's entirely possible the
dark matter does have some of these properties
that you could reverse in which
case there could be both dark matter
and anti-dark matter in our universe.
>> Okay.
So again, we got a couple
of experimental questions.
Why did they bury the labs
underground, why the sensors underground?
>> Yeah. So if you're looking for dark
matter, you have to build detectors
that are extremely sensitive, they have to tell
when a single dark matter particle
comes and hits one of your atoms.
So there's not a lot of room
for noise to be on top of that.
And if we did an experiment like that
on the surface of the earth cosmic rays
or energetic particles from space would
constantly be hitting your detector,
giving you false signals of dark matter.
Dark matter, on the other hand,
can pass straight through the earth
without it knowing it's there and
easily reach your underground lab.
The analogy I like to use is if you were
-- you know, try to listen for a pin drop,
I'd be pretty hard to do in a quiet location.
But trying to look for dark matter on the earth
surface is like trying to listen for a pin drop
at a Metallica concert, it's
the wrong place to do that.
You want to go as deep underground as you
can where things are as quiet as possible.
And if dark matter -- if you're lucky enough to
have dark matter start hitting your detector,
you'll have a chance of detecting
it and for what it is.
>> Okay. Here's a general question, Dan.
Is understanding black holes the biggest mystery
to understanding our universe
currently, if not what is?
>> Well, black holes certainly offer a way to
gain insight into many of the most exciting
and profound questions in physics.
Probably if you did a survey of
physicists today the answer --
the thing that they would say they want
to know the most is how general relativity
and quantum physics can be merged
together into oneself consistent theory.
We call this a theory of everything at
the risk of being a little over sweeping,
but that's what we call such a theory.
And we don't have any particularly
compelling candidates for that at this point.
I mean people have worked a lot in string
theory and things but we really don't know
if that has anything to do with
our universe, it may or may not.
And black holes are one of the ways in which
we know the universe uses quantum physics
and general activity at the same time.
So if we really could learn -- if we had a
black hole that we could study in the laboratory
and in detail we could learn
things about quantum gravity
that would help us build a theory like that.
So yeah, I would argue that black holes
are a profound importance and they're one
of the avenues we have towards these
really big fundamental questions
about our universe and how it works.
>> This goes back.
Why isn't the microwave background uniform?
That is the picture you showed
was not just all blue.
Why is it not uniform?
>> So I want to point out that it
is almost exactly uniform, okay?
So yes, I showed Apache map before,
but the hottest and coldest spots
on that map are only hotter or
colder than the average temperature
by about one part in a hundred thousand.
Okay. So extremely uniform in temperature
with some tiny variations upward and downward.
And the reason for those fluctuations is that
when those atoms are forming 380,000 years
after the Big Bang, there were parts of
the universe that were slightly more dense
and slightly less dense, and
those variations in density led
to those photons being a little
hotter or a little colder on average.
>> Okay. Does Planck's time have anything to
do with the first moments of the universe?
>> So when we -- this goes
back to the question --
the answer I gave to the
question about black holes.
When we take the theory of general relativity
and what we know of quantum physics,
those two theories seem like they could play
together pretty well and remain consistent
with each other up to a temperature that
we think the universe may have been at,
at 10 to the minus 43 seconds after the
Big Bang or Planck's time, the Planck time.
We know that these one or both of these
theories has to break down at that point.
So if we ever want to understand what
Stephen Hawking and Roger Penrose had in mind
with the Big Bang singularity something that
happened even before inflation, for example,
we would need a theory of quantum gravity
that would describe the laws of physics
as they pertain to that extremely, extremely,
extremely early epic of our universe's history.
And at this point we do not
have a theory and anyone
that tells you what the Big Bang singularity
was really like is either misleading you
or just speculating a little
more freely than I'm willing to.
>> Okay. Why does dark matter -- why
is dark matter affected by gravity,
but apparently not by light
and other interactions?
Does gravity somehow supersede all
the other forces and interactions?
>> Yes, we understand gravity in
the context of Einstein's theory.
Gravity is something that works on absolutely
every form of energy without exception.
There's something called the equivalence
principle which is built into it.
And basically what everything boils down
to here is when you put any form of energy
and space including matter but any kind of
energy, it changes the geometry of the space
around that point and that curvature of space
it leads to the phenomena we know as gravity.
So no matter what dark matter is it
should experience gravity the same way
as anything else.
Now, when it comes to the other forces we know
about in nature including
the electromagnetic force,
which is what allows you to
see something with light.
Not everything experiences that force.
In fact, the only things that experience that
force are particles with electric charge.
So an electron or a proton
which carries electric charge,
these are things that interact with light.
But particles like neutrinos, for example,
which do not carry electric charge,
do not directly interact with light and
this is one of the reasons why neutrinos are
so difficult to study and require such enormous
detectors like the Doom detector that you saw
in the video preceding this lecture.
>> Do we have any other means of finding out
the composition and the details of the universe?
But before 380,000 years to your point --
do we have other methods other than
the cosmic, you know, the radiation?
But is -- can gravity waves help
us here go beyond that point?
>> Yes. So at present, the only tools you
really have are the cosmic microwave background
and the light element abundances which get us
to a few hundred thousand years
and a few seconds respectively.
But using gravitational wave
detectors, we hope to be able to learn
about much earlier times in the future.
So far we do not have the -- we
have gravitational wave detectors
but not at the level of sensitivity
we need to study the universe.
What a gravitational wave is, is literally
an oscillation in space and time itself.
A wave propagating through space in which
the distances between different points
of space oscillate back-and-forth
as the wave pass through it.
It's an actual vibration of space and time.
And we have detective gravitational
waves from very specific kinds of events
where black holes merge into one
another or in some cases neutron stars.
And those sorts of compact mergers have led
to gravitational wave singles we've been able
to detect especially with the series -- the pair
of gravitational wave detectors we call LIGO.
Now in the future, we hope to
deploy gravitational wave detectors
that can detect gravitational waves.
It's a very different frequency ranges
and very high sensitivities in space.
And in particular, we have something
called the LISA detector we hope to deploy,
and then other things after that as well.
And with some of these detectors, it may
be possible to detect gravitational waves
that were produced in the first
fraction of a second after the Big Bang.
Gravitational waves might tell us about certain
kinds of base transitions or other events
that we currently don't know about, but
that may have taken place very early
in our universe's history.
>> Can dark matter be related
to information content?
As space expands is -- there's more empty space.
Can you say something definite about
how a space if it contains no matter?
Is there information there?
I guess this is sort of an entropy question.
>> Yes. So the short answer is any
time you have any objects in space
and I'm being deliberately vague but
that includes particles, or you know,
whether they'd be of dark matter
or photons or anything else.
And maybe perhaps space itself, in
fact, I'm pretty sure space itself.
There is information stored in all that,
there's entropy associated with that.
We think black holes have entropy proportional
to their surface areas, for example,
and we think of black holes just being
space and time curved in a particular way.
So yeah, I assume that dark matter whatever
it is has information content stored
in it and therefore entropy.
There are a number for profound questions
related to entropy and cosmology.
Sean Carroll wrote a very nice book discussing
some of these problems, in particular,
is a question of how our universe wound up
in the very specific state it started out in
from an entropic perspective
shortly after the Big Bang.
And I don't think there are any particularly
good answers to this question, yeah.
>> Okay.
Here's sort of a -- here's maybe a little
political one here, but here we go.
Several decades ago topological defects
such as cosmic strings, domain walls,
emerging grand unified theories were very
popular, right, superstrings, and so forth.
What is the current status of such phenomenon
and could they hold any clues to the resolution
of the problems you are discussing
as were unsolved?
>> Yeah. So the person asking
the question is absolutely right.
When I was in grad school, you know,
almost 20 years ago I was kind
of not exactly taught as fact.
But I was taught to fully expect
that when we figured everything else
when the dust settled we would learn
that there's a grand unified theory
that combines all the known forces of
nature into something neat and compact.
And it would include supersymmetry, this theory
that is very popular at that time and all --
and dark matter will come out of
it and this will come out of it
and all these things would
form a neat compact puzzle.
But as time has gone on that
attitude has changed quite a bit.
Most physicists today I would say go into these
questions with a lot more humility, we think,
you know, there might be a grand unified
theory, there might be supersymmetry
but there might not be in -- very likely
there's a bunch of stuff along the way
that we don't currently understand
or aren't envisioning.
I'm somewhere in between personally, I
think there's a lot that's very compelling
about supersymmetry and a lot that's very
compelling about grand unified theories.
And I think it's more likely than not that a
lot of that stuff is going to be manifested
in nature, but at the same time, I'm not
nearly certain about these things as I was
in grad school or as my professors
were in grad school.
There has been a palpable shift in attitudes
on these questions as the datas came in
and frankly not confirm some of
the predictions of these hypotheses
that were so popular at the time.
>> Well, I tell you what, Dan, since
we sort of got you talking on --
in a personal way, but let me go
farther with a couple of questions here.
What do you think the universe is like?
Is a multiverse or something different?
And then I'm just going to pile it
on, so you can expound it for a bit.
What made you want to study cosmology
and how does this questioning
actually affect the common man?
So there you go, you could
pull it out for 20 minutes.
>> All right, right.
So I'm going to take one piece at a time here.
So I'm going to start with --
>> -- Okay.
What's the universe like, a
multiverse or something different?
>> Yeah. So I'm not going to say I know there's
a multiverse or we prove there's a multiverse
or something like that, none of
those statements would be responsible
and they don't reflect my view.
But I think it should be one's default
position that probably is a multiverse.
When I think back at human history and
prehistory, I picture a bunch of people living
on an island somewhere and they're
wondering if there are other islands.
And of course, they think there isn't
because all they've ever seen
in a given direction is water.
They think of their island as unique, they
think of their island as the universe.
Eventually, they either get visited
from travelers somewhere else
or they travel somewhere else and
they learn that they were mistaken
and they get used to the
idea of their lost lives.
Later, some descendants of theirs perhaps
are contemplating the cosmos and they look
up at the sky and they see various
astronomical bodies, the Sun,
the Moon, things they call planets.
And they deduce that all of those things
are kind of alike, but the thing they live
on the earth is unique, everything
moves around them.
The Earth is not only center
of the universe but it's made
of different stuff and is completely unique.
It is the universe for all intents and purposes.
Of course, people like Copernicus and Galileo
come along and show that's not really true
and in fact, the Earth is just one of
many planets and orbits around the Sun.
After that people like Giordano Bruno start
to speculate that maybe those things out there
that we call stars are just like
the Sun, but a lot farther away.
That was not a very popular
idea at the time for that
and other positions Bruno was
burned alive at the stake.
So you can see why a lot of people might
not have proposed that at the time.
But the short story is, they -- over
hundreds of years accepted the --
human beings accepted this and learned
that even our solar system isn't
unique, it's not even special.
As recently as the 1920s, astronomers were still
arguing whether the Milky Way was the universe
or just one of many, many galaxies
that populate our universe.
And we know today there's something like half
a trillion galaxies in the observable universe.
So I think the step -- the
steps I've just described are --
a natural continuation of those steps would be
to say, well is the particular patch of space
that we occupy and can observe with
our telescopes, is that the only one?
Well, I don't see any reason
to think it would be.
If you have one universe that exists
they're probably a very, very large number.
All right.
So that's my answer to question one.
Question two, is why do I study cosmology?
I study cosmology because it's the most
interesting thing I've ever encountered.
I didn't -- I wasn't the sort of young
person who knew I wanted to be a physicist
or a scientist from a young
age, I had no intention
of studying science when I went to university.
And but at some point, I wound up taking
one-quarter class of modern physics,
where I learned about quantum
mechanics for the first time
and I learned about the theory of relativity.
And those were the two most exciting
things I'd ever learned about anywhere.
And I immediately changed majors and became
a good student more or less overnight.
I was a very poor student before
learning about those things.
And a very diligent, determined
student after finding
out those things could --
existed then I could study them.
Since then I've fallen in
love with a lot of branches
of physics, but none more than cosmology.
Trying to understand what our universe as a
whole is like, how it came to be the way it is,
what forms of matter and energy can be found in
it, and most importantly how the universe began.
These are the questions that still thrilled me
to stay and make me feel so lucky to be able
to think about and ponder as a profession.
Was there another question, Tom?
I can't remember.
>> No. Okay.
So yeah, I think that covered it.
Well actually the final one was the one that
gets asked a lot and I'll throw it to you was,
how does all this affect the common man?
>> Oh. Well, it really depends what
perspective you take on that question.
If I'm being honest, I am not doing cosmology
because it's going to lead to a better mousetrap
or any other technological applications.
There are legitimate arguments based
on economics and based on technology
for why we should fund things --
fundamental science including cosmology.
It does lead to a lot of spinoff technologies
that's been demonstrated to be true
and it convinces a lot of talented young
people to learn a lot of skills that turn
out to be very useful in other endeavors.
So there are good practical reasons
to do this, that's not why I do it.
I do cosmology because it --
the questions that we raise
in cosmology keep me up with
excitement at night.
It's the sort of thing that when I'm having
a beer with my friends I want to talk about.
No matter how many times I've talked about it
before it's the sort of thing I want to talk
about because it's exciting and it's thrilling.
And when I look back in the
history of humankind,
certain moments in intellectual history
that I think are the most worth celebrating.
Galileo producing arguments for why the
earth is not at the center of the universe
or Newton coming up with a unified theory of
the thing we call gravity and planetary orbits.
Or James Clerk Maxwell, explaining that
magnetic fields are really nothing more
than moving electric fields,
or name their thing of choice.
All of these things are the sort of things that
human beings will be celebrating for as long
as they exist on the earth
as enormous, you know,
landmarks crowning achievements
in the history of humankind.
I want to be there when those landmarks
get crossed, I want to understand them,
I want to bask in the glory of human
accomplishment at those moments.
>> Okay. So I'll tell you what, since we
brought you in one direction let's add --
here's a couple of sort of
straightforward bread and butter.
So how far can a photon be transmitted before
it dissipates so much that it can't be detected?
How far can a photon travel
before it's undetectable?
>> It's basically limitless.
So those photons that we observe in the
cosmic microwave background have been passing
through space at nearly straight
lines with undergoing next
to no interactions for 13.8 billion years.
And if you let them keep going
they would still be doing
that in another 13.8 billion
years or 13.8 trillion years.
The one way in which they are changed
substantially as they travel through space is
as the universe expands the
space expands the wavelengths
of those photons expands
with it, they get stretched.
And because the energy contained by a photon
is inversely proportional to its wavelength
that means those photons
lose energy as space expands.
So those universe -- those photons when they
were created were very energetic and are very,
very low energy today, but they're the
same photons and they've been traveling
with really no intervening interactions to speak
of throughout the entirety of cosmic history.
>> Okay. A couple of more and
then we'll let you go, Dan.
Where did the top and charm quarks go when
the universe moved into its current era?
Where -- you mentioned that they were there
in the beginning and they're not there now.
Where did they go?
>> So I also mentioned that any kind of energy
can transform into other kinds of energy
and in the early universe
this was going on a lot.
So prior to a trillionth of
a second after the Big Bang,
if any two particles collided
let's say two photons collided,
those two photons might be
transformed into a top quark
and an antitop quark, or
any number of other things.
But that's just one of the many
things they might transform into.
As the universe cooled those
photons would collide wouldn't
on average not have enough energy anymore
to make new top quarks and antitop quarks,
because top quarks are very heavy and
therefore require a lot of energy.
Meanwhile, those top quarks are
colliding with antitop quarks
and disappearing becoming other
forms of energy like photons.
So as the universe expanding cooled,
the heaviest things became more rare
and eventually very, very scarce.
And slowly the heaviest and second heaviest
and third heaviest particles became increasingly
rare until we're left with the array
of particles that we find in our universe today.
>> Okay. So here we go.
Is the Higgs boson the fundamental
unit of matter?
>> Well, the Higgs boson is an extremely
important particle in the way that it interacts
with other forms of matter and makes those forms
of matter behave in the way we observe them to.
In particular, if it weren't for the Higgs
boson a lot of the forms of matter and energy
in our universe that we know and
love would not have any mass.
And that would make them
behave very differently.
Like electrons, for example, we
believe electrons have mass only
because of the way they interact
with the field of the Higgs boson,
the Higgs field, which permeates all space.
If the Higgs did not exist, electrons would
be massless and therefore they would travel
at nearly speed of light, they
couldn't form things like atoms,
our universe would be entirely unrecognizable.
So I don't know if I would use the phrase
that the Higgs is the fundamental constituent
of matter, form of matter,
or something like this.
But it is a fundamental facet
of our universe and without it,
our universe would look nothing like it does.
>> Okay. Last question, Dan,
and then we'll let you go.
And here's one it's sort of
a philosophical question.
Should the multiverse idea "bother" us?
That is -- and bother is in quotes,
because it may not be possible
to detect any other universes
and confirm the matter.
And you know, the classic case of, you know,
what's a scientific theory if
you can't confirm or deny it?
What -- ?
>> It's a great question.
One that I think very reasonable and intelligent
people on both sides have expressed opinions.
So I'm lucky enough to be able to teach a
class of the University of Chicago from time
to time called Philosophical
Problems in Cosmology.
This is one of my favorite ones to bring up.
And we read a bunch of stuff from a lot of
smart people arguing both sides of this issue.
My view, however, is that
in principle the existence
of a multiverse could be
a scientific proposition,
in that there are things you could
potentially measure or observe
that would make you think the existence of a
multiverse is more or less likely to be true.
That to me is the real definition
of a scientific proposition.
Something that some sort of empirical
information could have bearing
on the likelihood of it being true.
And for example, prior to
discovery of dark energy,
Nobel laureate named Steve
Weinberg wrote a paper saying,
"If there were a large multiverse
you would naturally expect there
to be roughly this amount of
dark energy in the universe."
And then about 10 years later we measured
it and within a stone's throw anyway,
there is the amount of dark energy
that Steve Weinberg predicted.
Now, this doesn't prove there's a multiverse,
I don't think Weinberg or
anyone else would say that.
But that measurement makes me think it's
more likely that there's a multiverse
than I thought -- would have
thought prior to that measurement.
So maybe between this and several other
kinds of empirical data that we might collect
over the years, decades, centuries, or millennia
ahead, we could become increasingly convinced
that a multiverse exists, and in that
sense that hypothesis is a scientific one.
>> Okay. So again, we'll
let you go with that, Dan.
Thank you very much for the wonderful talk.
>> My pleasure.
>> Again, thank you.
>> Goodnight everyone.
