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Comment by grishka | Hacker News Reader

And if the universe was much denser, doesn't that imply that all that matter affected its surroundings gravitationally? And as we know, time runs slower near large masses. And when something falls into a black hole, according to our very own theories, it would also red-shift because of the black hole's gravitational pull without anything having to expand.

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No, it implies it expanded in the meantime. We can see that it was a hot plasma up until 300k years after the big bang. This isn't some redshifted illusion, the matter was literally packed so densely and thus so hot that it was in another aggregate state.

Don't get hung up on redshifts for evidence of the big bang. The CMB is the real smoking gun. Read up on it, it's entirely worth it. I can recommend Simon Singh's book "Big Bang".

There is also a plethora of other probes that in concordance all point to the same thing: that the universe is almost 14 billion years old and expanded from a very hot, dense state. It's settled science, really.

Speaking of the big bang, how did time work back then? :)

It's cool to say "in the first milliseconds of the existence of the universe X and Y happened", but how did time supposedly run as usual while everything else was on the fringe of our understanding of reality? There don't seem to be any answers to this (or I haven't looked thoroughly enough) but it feels like a very important question that's always overlooked by everyone talking about this.

Yeah, it is overlooked because the real answers are 'hidden' behind a lot of graduate level math. And most people don't really want to learn a bookcase worth of math first to talk about it, but they talk all the same.

Like, if you'd like to really dive into it then you're going to need to go through a lot of textbooks first.

If you are moderately familiar with multi-variable calc, then here is a good book to get started down the GR hole: https://www.amazon.com/Mathematical-Methods-Physicists-Compr...

Suffice to say, yes, there have been a lot of grad students that have the exact same questions and issue that you currently have. Further, once they have reached the end of the mathematical education required to understand how space time works in the first few minutes of the universe, they focus those questions into the issues we have with inflation. Those issues mostly come from our lack of understanding about how GR and QM interact, so the first 10e-43 seconds or so. At least, that is my understanding. Physicists are welcome to tell me how dumb I am right now!

(former) Physicist with focus on cosmology here. Your reply is one of the sanest in this thread.

There's a lot of attempts at investigating those questions. Here's a couple of pages I'd recommend to peruse:

https://en.wikipedia.org/wiki/Cosmic_time

https://en.wikipedia.org/wiki/Chronology_of_the_universe

If you're into podcasts at all, I'd strongly recommend Crash Course Pods: The Universe. The first (full) episode goes into detail on that first fraction of a second in our universe and it's pretty enlightening without being to thick on the math.

> doesn't that imply that all that matter affected its surroundings gravitationally?

It did; it caused the expansion to decelerate. That was true until a few billion years ago, when the matter density became smaller than the dark energy density and dark energy started to dominate the dynamics.

It's really trippy to think about how hawking radiation becomes 'real' once its sufficiently 'far' away from a 'strong' gravitational well, and how this can be thought of as a Doppler shift giving real physical presence (in that we can interact and be affected by ) to what was once a 'virtual' particle

I think scienceclic does a good job visualizing this, but end of the day I can't see a way to distinguish event horizons regardless if they're a black hole or the distant past/big bang.

https://youtu.be/isezfMo8kWQ?si=9wGliV-Qo1bXCTRy

Specifically look at the relativity of the vacuum section, which builds to a great insight at around 5:45

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