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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