#piningmarten

[auroral-melody]

I've been thinking of something for a while and I'd like your opinion on it... what if instead of the universe expanding from a single point with the big bang, the speed of light was infinite until the big bang at which point it became a finite number and has been decreasing since.

So this ask is fully from May 2018. I wrote up a whole thing about it, and then ended up never posting it because I wasn’t very confident in my answer.

4 years later I had done a research project in particle cosmology and more or less concluded that no one actually knows anything ever, physicists should be taught more group theory, and that spinors are evil.

Anyway, 2 years after that, here we are. I doubt you even remember sending this. This ask is honestly still out of my area of expertise. Yes, I’m an astrophysicist, but I focus on stars which means my problems are so different 😂 If any cosmologists follow me then I welcome corrections.

Anyway, I skimmed my answer I wrote 6 years ago and edited it. Shoutout to younger me to doing their research alright – I mostly hit the main points lmao.

Firstly, to clarify, the Big Bang is difficult to discuss in terms of space because it’s fundamentally the expansion of space. We don’t have any other reference point, so as far as we’re concerned, there’s no single point that the universe expanded from, because it expanded everywhere.

Anyway, in general, we consider the speed of light to be constant because that’s been our base assumption for the Entirety Of Physics Really.

But if we dismiss all of modern physics and reinvent it, there is some interesting speculation to be had regarding the speed of light.

Essentially, when we look at all our maps of the universe, we see quite a bit of “isotropy” – that is, generally, it’s about even in density. (I’m talking on a scale much bigger than galaxies – think galaxy clouds, local groups, etc.) In order for this to happen, light/energy has to have traveled to all these places. But with the current speed of light, that’s simply not really possible given the age of the universe!

So there’s two theories to explain this:

1) Inflation. This is the most common one. The theory is that the universe kinda evened out when it was little, and then expanded.

2) Variable Light Speed. This is closer to your suggestion, and also something posited by João Magueijo, of Imperial College London, and Niayesh Afshordi, of the University of Waterloo in Canada, as well as Jean-Pierre Petit and John Moffat. In this theory, the speed of light was infinite, or at least much higher, at the Big Bang, so that light could propagate fast enough to get everywhere in the universe.

In general, VLS (or Variable Light Speed, or VSL Variable Speed of Light) isn’t widely accepted in physics. It’s not like it’s been disproven or anything; we just don’t have evidence for it.

If we do find evidence for VSL, we need to totally rework physics. G.Ellis, according to Wikipedia, explained that in order for VSL to work, we

must redefine distance measurements (the meter is the distance light travels in 1/299792458 seconds, by definition)

must provide an alternative expression for the metric tensor in general relativity (i.e., “we have to try to redefine what space is”)

might contradict Lorentz invariance (the laws of physics are the same no matter where in the universe you are. it’s a fundamental assumption, and changing it will break pretty much everything in physics)

must modify Maxwell’s equations (explain why electromagnetism is a thing. which is hard. light is by definition an electromagnetic (EM) wave, so if we mess with light is screws things up)

must consistently explain other physical theories too. (gravity still has to exist and make sense with your math)

The theory proposed by João Magueijo and Niayesh Afshordi is, tldr, really complicated, and poorly understood. If it is right, we’d kind of have to try again with the whole physics thing and redefine a bunch of stuff and do some serious math.

People who like VSL as an idea are waiting to see what super sensitive experiments (about the ~spectral index~ of the universe) have to say about their predictions.

Thanks for the ask!

[piningmarten]

url change

thisis-isa --> piningmarten