If there's a question that you guys never
expected me to ever ask, it might be "have
astronomers disproved the Big Bang?"
And I understand why you might think that.
After all, in my videos, I describe what scientists
know about the most cutting-edge topics in
physics research.
So, perhaps I should be up front and give
the bottom-line answer.
No.
Scientists have most certainly not disproved
the Big Bang.
Sorry if you hoped to hear that they have.
But it's certainly true that there is a scientifically-fascinating
disagreement between two predictions of the
Big Bang Theory on how fast the universe is
expanding.
And those disagreements, while small, are
a big enough deal that astronomers are thinking
about whether the Big Bang Theory will require
modifications and extensions.
Or maybe the whole situation might go away
with an improved understanding of the existing
theory.
It’s all very exciting.
So let me explain what the mystery is.
First the basics.
There are four important points to focus on.
They are that the Big Bang had a beginning,
the universe is expanding, the universe has
evolved, and gravity affects the rate of expansion.
While the Earth is not at the center of the
universe and not a unique location where the
Big Bang occurred, it is at the center of
our visible universe.
That means that in any direction we look,
we see the same behavior, which is that distant
galaxies are moving away from us, and they
are moving away from us at a specific rate.
The name we have for this rate is called the
Hubble constant.
Basically, the Hubble constant tells us how
fast galaxies are moving away from us as a
function of their distance.
Let’s start by talking about the Hubble
constant in round numbers.
It's about 70 kilometers per second for every
megaparsec of distance the galaxy is from
the Earth.
A megaparsec is just 3.3 million light years.
So a galaxy one megaparsec away from us is,
on average, moving away from us at 70 kilometers
per second.
A galaxy two megaparsecs away is moving at
140 kilometers per second.
Three megaparsecs means 210 kilometers per
second, and so on.
The scientific fireworks arise when scientists
try to accurately measure that number in two
different ways.
They should agree.
But they don’t.
The first way measures the conditions of the
universe when it was very young and then calculates
what the expansion rate should be now.
The second simply measures the rate now.
Let’s take a quick look at both methods.
Scientists can’t use telescopes to measure
the moment of the Big Bang itself.
But in 1964 they used radio telescopes to
look at the cosmic microwave background or
CMB, which is a picture of the universe when
it was about 400,000 years old.
Now, 400,000 years old sounds pretty old.
I mean, that’s even longer than the entirety
of Betty White’s television career.
But, as you can see here in this picture,
the CMB is pretty close to the time the universe
began, so it’s a good way to see the universe
when it was very young.
The CMB is nothing less than a measure of
the temperature of the Big Bang, although
it has cooled a great deal over the last 14
billion years, and it's very uniform.
The temperature is essentially the same in
every direction.
But, if you look at it very carefully, you
see that there are tiny variations in the
temperature, with some places being slightly
hotter and some are colder.
Those variations are about only about 0.001
percent, but we can measure them.
And, when we do that, we see that the temperature
pattern is a little splotchy.
Astronomers using a facility called the Planck
telescope can measure the distance between
hot or cold patches and use that measurement
to make a prediction of what the Hubble constant
is today.
And, remember, the Hubble constant is a measure
of how fast the universe is expanding in the
present day.
Using these observations of the early universe,
astronomers say that the Hubble constant is
67.4 plus or minus 0.5 kilometers per second
per megaparsec of distance.
That’s a pretty precise measurement, with
an uncertainty of less than one percent.
So that’s the first measurement.
The second measurement is more straightforward.
Using the Hubble space telescope and other
facilities, astronomers measure the distance
to galaxies out to a distance of a few hundreds
of millions of lightyears or so, and then
they also measure their velocity.
From that, you can work out the Hubble constant.
As of this moment, the most precise measurement
of the Hubble constant using this method is
74.0 plus or minus 1.4 kilometers per second
per megaparsec.
That’s a 1.9 percent uncertainty.
We can compare those two numbers.
One of them has an uncertainty of 1.9 percent,
while the other has an uncertainty of 0.7
percent.
Yet, they disagree by just shy of ten per
cent.
And that is what scientists are worried about.
These two numbers disagree.
And they shouldn’t.
So, does this mean that the Big Bang has been
disproved?
No, of course not.
Nobody seriously questions that the universe
began 14 billion years ago and has been expanding
since.
The data supporting that idea is simply overwhelming.
But a disagreement can point to the need to
tweak an existing theory.
So what could it be?
Well, first we need to acknowledge that this
disagreement is between two different experiments.
It's possible that one or both of the measurements
is wrong.
One of the groups could have made a mistake
or overlooked some aspect of the data that
they haven’t taken into account.
So while we must admit that possibility, the
two different groups are both excellent.
There are many researchers involved in the
two measurements, but the flagship teams,
using the Planck and Hubble telescopes are
among the best in the world.
They’re almost as good as Fermilab researchers.
Almost.
So we can conclude that we have to take the
discrepancy seriously and we can be assured
that these researchers are checking and rechecking
their data for possible mistakes.
And, if you have any ideas, I’m sure they’ll
look into them, or, more likely, they already
have.
But what if the discrepancy is real?
What if these two ways of measuring the same
thing are actually different?
What would that mean?
Well, at its core, it means that when you
take the conditions of the early universe
and project them to today, you get the wrong
answer.
So that means that there is a phenomenon that
we don’t know about that is overlooked in
our current calculations.
Astronomers don’t believe that they have
overlooked anything in the usual physics theories.
If they did, then they wouldn’t be able
to explain things like the distributions of
galaxies in the universe.
But it's important to remember that ordinary
matter only makes up five percent of the matter
and energy in the universe.
The remaining 95 percent is made of dark matter
and dark energy.
Dark matter is a form of matter that experiences
gravity, but it doesn’t give off any light,
and dark energy is a type of gravity that
repels, rather than attracts.
So, one possible explanation is that dark
matter experiences interactions with ordinary
matter beyond the known gravitational interaction.
We know that dark matter doesn’t experience
any of the other known forces, specifically
electromagnetism or the strong or weak nuclear
forces.
But perhaps there is an unknown force that
is weak enough that we haven’t discovered
it, but it connects dark matter and ordinary
matter.
Another possibility is that dark energy is
more complicated than we currently think.
Astronomers believe that in the very early
universe, just a fraction of a second after
the Big Bang, something like dark energy caused
the universe to expand in what is called inflation.
Then the effect of dark energy somehow turned
off until the universe was about nine billion
years old and then became important again,
causing the expansion of the universe to begin
accelerating.
But perhaps there was another phase early
in the history of the universe- say around
when the cosmic microwave background was emitted-
that dark energy appeared briefly and then
disappeared again.
That would cause the universe to be expanding
faster than we think it is, and that could
possibly explain the tension.
And there are other possible explanations
and we simply don’t know which, if any of
them, is the answer.
Indeed, we aren’t even 100 percent certain
that the discrepancy is real.
But, you know, that’s frontier science for
you.
When you’re in the middle of it, it often
takes a while to figure these things out.
On the other hand, even if this discrepancy
is real, it is certainly premature to claim
that there is anything seriously wrong about
the theory of the Big Bang.
I mean, let’s get some perspective.
This is a 10% disagreement in a single parameter.
At the big picture level- the one where the
universe was smaller and hotter and is now
expanding- the Big Bang Theory is entirely
solid.
But solid isn’t perfect and we scientists
are constantly digging into the data looking
for clues that will help us advance our understanding
of the rules of the universe.
It’s a bit of a messy business and it can
take some time, but it’s great fun and we’ll
let you know when we figure it out.
So that topic was fun.
Scientists love discrepancies.
They’re like a loose thread in a sweater.
Tug at it, and the whole thing can unravel
and we can knit a new and better sweater...
or physics... or something like that.
I don’t know.
Metaphors are hard.
I’m a physicist, not an English major.
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