Did Time Start at the Big Bang?

SPEAKER_1 (0:00)

This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (0:30)

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SPEAKER_3 (0:42)

YOLO! YOLO!

SPEAKER_2 (0:43)

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Guard against wild money moves with PNC Bank. Brilliantly boring since 1865

Matt O'Dowd (1:00)

Thank you to LastPass for sponsoring PBS Digital Studios. Our universe started with the Big Bang. But only for the right definition of our universe. And started, for that matter. In fact, the Big Bang is probably nothing like what you were taught.

A hundred years ago we discovered the beginning of the universe.

Observations of the retreating galaxies by Edwin Hubble and Vesto Slipher, combined with Einstein's then brand new general theory of relativity, revealed that our universe is expanding. And if we reverse that expansion far enough, mathematically, purely according to Einstein's equations, it seems inevitable that all space and mass and energy should once have been compacted into an infinitesimally small point. A singularity. It's often said that the universe started with this singularity. And the Big Bang is thought of as the explosive expansion that followed. And before the Big Bang singularity? Well, they say that there was no before, because time and space simply didn't exist. Now if you think you've managed to get your head around this bizarre notion, then I have some bad news. That picture is wrong. At least according to pretty much every serious physicist who studies the subject. The good news is that the truth is way cooler. At least as far as we understand it. Now before a certain crowd starts with all the scientists keep changing their minds they don't know anything, or the Big Bang theory is just a theory, let me be very clear. The evidence for a hot dense early universe is practically incontrovertible. The cosmic microwave background is a direct line of sight to the universe as it was only a few hundred thousand years after the hypothetical beginning of time.

We can see pretty much directly that all space and matter in the universe was once crunched at least a thousand times closer together.

There's also the relative abundance of simple elements, hydrogen and helium in particular, whose ratio is exactly what we expect if the entire universe was a dense billions of degrees nuclear furnace for the first several minutes of its existence. In fact, there's powerful evidence that we should not rewind Einstein's equations that far, at least without introducing some very new physics.

For one thing, there's also convincing observational evidence that the time before around 10 to the power of negative 32 seconds included a period of extremely rapid expansion called cosmic inflation. We've talked about the reasons we need inflation in previous episodes, and I'll come back to it in a bit. Adding that initial growth spurt solves a couple of the big problems with the Big Bang Theory, but it doesn't change the fact that rewinding the expansion of the universe, even at different speeds, still leads us.

SPEAKER_1 (4:00)

This episode is brought to you by Accenture. When your advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales, using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business.

The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (4:30)

PNC Bank brings you Call of the Wild Money Moves.

SPEAKER_1 (4:34)

Shh.

SPEAKER_5 (4:35)

Listen. Hey, guys.

SPEAKER_2 (4:37)

That's the sound of a multi-level marketing pitch.

SPEAKER_5 (4:40)

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SPEAKER_2 (4:43)

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SPEAKER_5 (4:46)

They are so essential.

SPEAKER_2 (4:47)

And then have all your friends buy essential oils.

SPEAKER_5 (4:50)

Are you more of a geranium or a lavender fan?

SPEAKER_2 (4:53)

Don't look her in the eyes.

Guard against wild money moves with PNC Bank. Brilliantly boring since 1865

Matt O'Dowd (5:00)

towards the t equals zero singularity. I'm going to come back to why we need to forget the idea of this singularity. Doing so will change the way we think about cosmic inflation and about the beginning of the universe. But before we kill the whole idea of the Big Bang singularity, we need to understand what we're killing. What does it really mean for all of space to be compacted into a single point? This idea is especially weird if the universe is infinite. Now the universe may or may not be infinite, but if we can understand this for the infinite case, then getting all of this for the finite case is baby stuff. At least by comparison. It's tricky to talk about the size of an infinite universe. Instead of the overall volume or radius, we talk about the size of an expanding infinite universe in terms of the scale factor. That's the distance between any two points in space at some moment in time relative to their distance at some other reference moment. That reference moment is typically taken to be right now. So the scale factor of the universe is currently one.