In December last year, science
teams at CERN's Large Hadron
Collider reported
a hint of evidence
of a brand new particle, one
that does not fit anywhere
within the standard model
of particle physics.
If true, this would be
the first clue of anything
beyond the standard
model from the LHC.
Remember, the standard model
is basically the periodic table
of fundamental
particles and forces
that represents our entire
current understanding
of the building
blocks of all matter.
Even the incredible discovery
of the Higgs bosun in 2012
was a confirmation of the final
particle of the standard model.
And that was expected.
If this is real, it may break
open entirely new pathways
to the research of the
fundamentals of nature.
So what was seen?
Well, let's first think about
how particle accelerators--
and especially the LHC-- work.
The LHC occupies a 27-kilometer
circumference circle
beneath the Swiss-French
border near Geneva.
A ring of gigantic magnets
accelerates protons
to up to 0.99999999
of the speed of light
before colliding them
from opposite directions.
The resulting collisions
can produce temperatures
of several trillion Kelvin.
So a tiny spot
for a tiny instant
resembles the state of the
universe at a trillionth
of a second after the Big Bang.
Under these conditions,
protons are obliterated.
Their energy is released
and reshapes itself
into new particles.
Many, many weird particles
come out of such a collision.
And some are so
hopelessly unstable
that they decay into high
energy light-- gamma rays--
before they ever
reach a detector.
We only know they ever existed
because the resulting gamma
radiation has an energy
corresponding to the mass
of the decayed particle.
The Higgs boson was
found because there
was a slight excess
in gamma ray flashes
at 125 gigaelectron volts above
the otherwise smooth spectrum
of gamma ray energies.
And now there's a
new bump at 750 GEV.
This suggests a new particle
with a mass much larger
than anything in
the standard model.
When it was first spotted,
this gamma ray excess
was just a little bump.
And it could have been
the natural result
of random fluctuations.
Those happen all the time.
We wouldn't have heard
anything about it
except that two completely
separate experiments using
separate detectors-- Atlas and
CMS-- both saw the same bump.
The significance of
the result is still
low, at around a sigma of 1.6
for CMS, and lower for Atlas.
That's nowhere near
the 5 sigma needed
for a confident discovery.
But particle physicists are
completely losing their minds.
Over 300 papers
have been submitted
to journals with a wide range
of possible explanations.
The broad family
of possibilities
include one-- dark matter.
There are theoretical
ideas of particles
that could cause a
bump at this energy,
and would also make pretty
decent dark matter candidates.
Two-- a gigantic neutrino.
More accurately, a very mess
of cousin to the neutrino,
predicted by supersymmetry
and as yet unproven,
but a pretty popular extension
to the standard model.
Three-- the big brother
to the Higgs boson.
So, a higher energy
vibration in the Higgs field.
Four-- the graviton.
A highly speculative
particle responsible
for the transmission of
the gravitational force.
But it's not yet even known
whether such a particle
exists, or is needed
to explain gravity.
Or five-- it's a composite
of other smaller particles.
Just like the proton is a
combination of three quarks,
this could be a much
more massive combination
of several quarks
and antiquarks.
There are links to references
to all of these possibilities
in the description.
There's no point choosing
between these options
until we verify the results.
The LHC is currently
offline for upgrades,
and starts up again
in June, following
a two-week delay due to a weasel
chewing through a power cable.
I kid you not.
At that point, the signal will
either solidify or vanish,
assuming no more
attacks by cute animals.
OK
Time for the solution to the
dark energy challenge question.
I'm just going to interrupt
Matt here for a second.
Sorry, Matt.
He's about to go full nerd
with the dark energy challenge
answer.
Before he sends you
to sleep, I want
to respond to your comments
on our Ice Age episode,
and make this
important announcement.
On Monday, June 13,
I'm doing a Reddit AMA.
We're going to talk about
everything space time.
So space and time,
really anything physics
or astrophysics.
We'll also talk
about my own research
in astrophysics, which is the
subject of a recent documentary
made by the American Museum of
Natural History, linked below.
But this is an Ask Me Anything.
So the sky's the limit.
Wait.
It's not even the limit.
How does this work?
You head over to the R/Science
subreddit that morning
and post your questions.
It will be the top three.
At 1:00 PM, I'll get
to answering these.
And we'll see how much
ground we can cover.
See you there.
OK.
Let's see what you guys had
to say about our episode
on the ice ages and the
Milankovitch cycles.
This ended up being a big
discussion on climate change.
But that's cool.
Astrophysics does have a
lot to say on the topic.
Bookmaker talks
about the connection
between solar activity
and Earth's climate.
And that's something we really
didn't touch on in our episode.
So it's true that
high sunspot activity
can increase solar irradiance.
And in the first
half of the 1900s,
increasing activity did increase
solar output by about 0.1
of a percent.
The consensus is that
this is much smaller
than anthropogenic factors.
But even ignoring
this consensus,
the correlation between
solar activity and warming
stopped a while ago.
As Bookmaker suggests, solar
activity is diminishing.
In fact, over the
past 35 years, it's
decreased while temperatures
continue to increase.
Xenion341 asks why the current
warming trend doesn't quite
track the current CO2 trend.
to the same degree that it does
in the paleoclimate record.
OK.
Well, temperature
is following CO2.
But the response takes time.
Global average
temperature has risen
by at least 0.6 degrees
Celsius, probably closer to 1
degree since around 1900,
following the CO2 increase.
However, the full
effect takes longer,
because it depends on
positive feedback cycles.
Reduced ice cover
reduces albedo.
Warming oceans and
melting permafrost
add more CO2 to the atmosphere.
In the paleoclimate
record, we see
that an increase in temperature
due to Earth's changing orbit
proceeds an increase in CO2.
So the feedback cycle
can start at either end.
And there will always be
a lag between the two.
But once initiated,
it doesn't really
matter whether the
CO2 increased first,
or the temperature
increased first.
Each drives the other.
Sugarshakerfly gently
asks whether we really
can be certain of the broad
extent of an effect, given
that we can't perfectly
accurately model every detail.
This is a tough one.
The fact is, we don't
need to be able to predict
with absolute precision
to be sure of the trend
and the severity
of a phenomenon.
Weather prediction
can usually tell you
that it will be hot tomorrow.
But it rarely nails
the exact temperature.
Similarly, completely
independent long-term climate
models differ in the details
of their predictions.
But almost all agree that the
current trends will continue.
By current trends, I mean
the increasing temperatures,
more severe droughts, shifting
climate zones, reduced ice
coverage, et cetera,
whose current effects
are very well-documented.
Sugarshakerfly also asks
why the Mesozoic was so bad.
Well, it wasn't.
It was great for
the organisms that
were perfectly adapted to it.
Most of those same
organisms went extinct due
to sudden and massive
environmental changes, probably
including climate change
due to meteor impact.
Sudden climate change
is very, very bad
for ecosystems evolved
for the current climate.
OK.
Back to [INAUDIBLE] Matt for the
dark energy challenge answer.
Take it away.
For the main question, I
asked you to figure out
how many times the
universe doubled
in size after dark energy first
started to show its influence.
And how many times
it would double
in size in the future
before matter no longer
has any significant
influence on expansion.
More precisely, for how many
past and future doublings
of the scale factor are
they both at least 10%
of the energy content
of the universe?
Let's think about a
giant box of space
that is expanding with
the rest of the universe.
We call this a co-moving volume.
To start with,
let's ask how large
this volume was when dark
energy only comprised
10% of its total energy.
So at that point in
the past, we know
that this equation is true.
Right now, 70% of the energy
in any volume in the universe
is in the form of dark energy.
So 70 parts dark energy
and 30 parts matter.
Now that looks like this.
Back in the day, it was
whatever the sum of dark energy
and matter was back then.
The overall amount of
energy in the form of matter
doesn't change, because
matter is spreading out
with the expanding volume.
Matter past equals matter
now for a co-moving volume.
But the amount of energy
in the form of dark energy
is proportional to the volume.
And the volume of the box
is equal to the length
of its sides cubed.
We can scale past
dark energy according
to the current dark
energy content like this.
And we can just use the
scale factor of the universe
as the side length of our box.
In fact, let's just use
R for the ratio of past
to present scale factors.
In fact, R is actually
what we want to find.
So our total energy
becomes this.
Put all of this into
our original equation
and rearrange, and
we get this equation
for the ratio of scale factors.
Plug in our current 70
units of dark energy
and 30 units of
matter, and we get
that the universe was
36% of its current size
when dark energy had
a 10% contribution.
But I asked you how many
doublings ago that was.
So you need to double
that 0.36 about 1.5 times
to get to the current
scale factor of 1.
You can take exactly
the same approach
to ask when in the
future matter will only
have a 10% contribution to
the energy in that volume.
It will happen when the universe
is larger by a factor of 1.57,
or about 0.7 of a
single doubling.
Add these two numbers
together, and you
get that matter and
dark energy both produce
a significant effect for
around two doublings.
That's tiny compared to
the 100 past doublings that
have happened since
the end of inflation,
and the infinite
number of doublings the
will happen in the future.
For the extra
credit question you
were asked to figure
out the number of years
that this corresponds to.
Now to do that, you need to
integrate the first Friedmann
equation over the
scale factors that you
got for the main
part of the question.
The answer is that
overall, the two influences
will be within an
order of magnitude
of each other for
around 15 billion years.
Now that may sound like a lot.
But we find ourselves right in
the middle of this very narrow
logarithmic window.
The solutions to both
parts of the question
are linked in the description.
If your name appears on screen
below me, you got this right,
and were randomly selected to
receive a space time T-shirt.
If that's you, email us
at PBSspacetime@gmail.com
with your address,
US T-shirt size,
and let us know if you want
a black hole orbit or an I'll
sign to anything I want T-shirt.
OK.
Blah, blah, math, blah.
What do I mean when
I talk about this
being a cosmic coincidence?
Is there something
about the tipping point
between the dominance of
matter versus dark energy
that makes the universe
more hospitable for life?
Or should we take
this as evidence
that our simple idea of a
constant dark energy density
is wrong?
Excellent questions
that cosmologists
are thinking hard about.
We're come back to
both of these ideas
in future episodes
of "Space Time."
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