How do we know the universe has always been expanding

I had a set of common beliefs on the universe shattered in a recent forum thread, and I’ve been doing a little reading to better myself. A thought occurred to me, and although I’m sure I’m not onto anything I want to see what the flaw in the logic is.

If I have my facts correct, Edwin Hubble first noticed that space itself was expanding. In recent years, physicists have concluded that the expansion rate is increasing, and is not a constant rate (or linear rate dependent on distance).

OK, now, here’s my thought. We say the universe is 13.7 billion years old, give or take. But, wasn’t this decision based upon the expansion of the universe? Play the expansion backwards and you have singularity in 13.7 billion years, so that’s gotta be the age. Now, if the rate is increasing, then when you play the tape backwards the rate of contraction would be decreasing! How do we know it ever was a singularity?

If that made no sense, say the universe is expanding at 74km/sec/pasec. But say a billion years ago it was expanding at 65km/sec/parsec. Maybe 10 billion years ago is when it started expanding and before that it wasn’t expanding at all.

I realize this leaves other issues like if the age is infinite, then hydrogen should be long gone. Protons would have decayed (maybe) etc. But unexplained caveats don’t disprove any hypothesis. So how do we know the rate of expansion was always above zero?

But I think that I have a pretty good understanding of basic astronomy and cosmology.
My (and my SO’s) initial opinion of the OP’s question is "Hmmmmm! That’s a very good question! :smiley:

When we look at distant galaxies, we’re also looking back in time. If the rate of expansion had varied significantly in the time since galaxy formation began, we would see anomalies in the red-shift of distant galaxies compared to their apparent brightness/angular size.

And prior to that we DO think that the expansion was briefly much faster. That epoch is called inflation. We believe this because, very simply, the universe is more uniform than it should be with a slow and steady expansion.

A very good “succint” answer, sir! Makes perfect sense to this layman.
Ignorance, successfully fought!

My SO and I, thank you.:smiley:

While Hubble’s Law was the first indication that the Universe is of finite age, and the reciprocal of the Hubble parameter is a reasonable back-of-the-envelope estimate of the age of the Universe, current measurements of the Universe’s age aren’t based on it, but rather on observations of the detailed structure of the Cosmic Microwave Background.

If I am grasping the OP’s question correctly it is being asked how we know, running the clock backwards, if the “compression” occurs at ever slower speeds then how do we know everything just doesn’t stop contracting before a singularity is reached.

Well, you’d need to come up with a mechanism that got everything, such as galaxies, moving from a standstill.

I can think of no mechanism for that except for the Big Bang itself which requires everything start at a singularity.

It hasn’t always been increasing. For the first 8 billion year or so, it was decreasing. The expansion was initially the residual motion left from inflation. After that, gravity was the main force operating in the universe and it was slowing the expansion.

However, about 5 billion years ago, dark energy started to overwhelm gravity as the major force and it is causing the expansion to speed up. Note that dark energy has been there from the beginning as a push for expansion, but it didn’t become the major force until expansion weakened gravity enough that it became stronger.

Interesting OP and thread.

Oops, I just realzed I used some misleading language. Not “residual motion”, but rather “residual stretching”. The Big Bang was not an explosion and I don’t want to give anyone the idea it was one. Everything (as in galaxy clusters) is moving away from everything else, but that’s due to space expanding, not from any relative motion.

At bottom, I think it’s just that ‘always expanding’ models are the simplest ones that fit observational data. However, there are alternatives in which the universe had a contracting phase in the past – but those must fit with observational constraints as well such that effectively, our universe must have been expanding for the past 13.7 billion years.

One such model comes from loop quantum cosmology, which predicts a so-called ‘bounce’ scenario: the big bang wasn’t the start of everything, but rather occurred at the end of the collapse of a previous universe, which under its own gravity contracted down to a very small (but finite!) size, at which point, gravity becomes repulsive, and everything starts expanding again. But from that point on, the expansion has to match the standard big bang model very well, in order to not be in conflict with observation.

Works for me, thanks again SDMB! You’ll probably see me ask another of these in a week or so… I’ve got one more question festering.