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Why do we always think of the universe as expanding?

Surely, from our perspective an expanding universe would look exactly the same as a universe in which everything is shrinking away from everything else? "Oh, but the red shift!" I hear people exclaim. Well how about this:

The speed of light is simply slowing down.

Hear me out. If the speed of light is a measuring stick against which all things are measured, and if the speed of light is slowing down, then it means everything shrinks and high frequency waves generated in the past are observed with a lower frequency today, which causes us to make observations that the universe is expanding.

What is wrong with my alternative?

Given this, if the speed of light were experiencing, say, exponential decay, wouldn't we expect to see the universe's 'expansion' accelerating? (which I thought was a relatively new and profound discovery)

Apologies if the question is silly, I've never made a formal study of physics and the universe, but this has always baffled me and I haven't been able to find an explanation

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  • $\begingroup$ What do you mean by "everything shrinking"? $\endgroup$
    – Sir Cumference
    Oct 1, 2017 at 17:30
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    $\begingroup$ This theory suffers more problems than you can address I think. For this explanation to work you also have to figure out the effects on gravitational interactions, for example, and how the quantum physics of physically shrinking matter works. For example, how does the emission spectrum of a physically shrinking atom change over time and how does that show up in measurements of cosmological light sources? Then all of the myriad changes would have to balance exactly to look like an expanding universe... which is overly complex when compared to a dark energy expansion theory. $\endgroup$
    – Asher
    Oct 1, 2017 at 21:57
  • $\begingroup$ @Asher, I see what you mean. Perhaps a better question would have been: If the speed of light were to change, would it be possible for all the other constants of the universe to scale in such a way that we wouldn't notice? $\endgroup$
    – Matt
    Oct 2, 2017 at 4:35
  • $\begingroup$ @SirCumference imagine that on your computer screen you can see 100 galaxies. As the universe expands the view on your screen zooms out so that the 100 galaxies stay in view. When you look at the screen, the galaxies will appear to shrink. How would you actually know if the galaxies are shrinking, or if they are moving further apart and the view is just zooming out to keep them in frame? $\endgroup$
    – Matt
    Oct 2, 2017 at 4:41
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    $\begingroup$ you are mistaken but I don't blame you because I fell for it too, scientists don't measure speed of light in vacuum but rather they declare it to be at that exact value! before we cry foul they do indeed use light to measure how long is a meter with an accurate clock... $\endgroup$
    – user6760
    Oct 2, 2017 at 8:50

2 Answers 2

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Surely, from our perspective an expanding universe would look exactly the same as a universe in which everything is shrinking away from everything else?

This is false. If everything was shrinking away from everything else, that would imply that the recessional rate of distance we observe due to this shrinkage would be independent of distances. Afterall, all galaxies are shrinking at the same rate so the rate they "recede" is only dependent upon their shrinkage rate.

However, we observer that the more distant an object is, the faster it recedes from us. This is the premise of Hubble's law and is only explained if we assume the universe is expanding.

Furthermore, there are many objects in our visible universe which are receding from us faster than the speed of light. This is only possible because these objects are not moving through space, but rather space itself is expanding. In your scenario, these objects are shrinking in space (I presume) and there's no possible way they could shrink fast enough to appear to be receding faster than the speed of light.

"Oh, but the red shift!" I hear people exclaim. Well how about this:

The speed of light is simply slowing down.

There have been people who have tried to suggest that the universal constants (e.g., the speed of light, the mass of an electron, etc.) are not actually constant and instead time-varying (albeit slowly). No such theory has ever been successful.

Again though, your idea suffers from a few problems, even if you assume it to be true. There are multiple kinds of redshift. Redshift from receding objects occurs because space itself is expanding and causing the wavelengths to expand as a result - otherwise known as cosmological redshift. In your scenario, the redshift would be caused by the (faster-than-light) recession itself (since the galaxies are physically moving away from us), also known as doppler redshift. You then complicate the problem by making the value of the speed of light change over time. This would cause light, at the moment of emission, to be redshifted both by the doppler effect and by the slowing of $c$. As the light travelled, the time-varying of $c$ would have no effect on the wavelength. In this scenario we would have long ago realized that moving objects get a double redshift, namely the doppler effect and the time-varying $c$ effect. We just don't see that.

If the speed of light is a measuring stick against which all things are measured, and if the speed of light is slowing down, then it means everything shrinks

Those two ideas don't relate at all. There's nothing suggesting the speed of light slowing down implies everything shrinks.

high frequency waves generated in the past are observed with a lower frequency today

As explained above, your proposal would cause the redshift to occur at the time of transmission, not en route. So there wouldn't be "high frequency waves generated in the past" They would have been emitted as a lower frequency. Which itself is troubling because how then do you explain the CMB changing frequency over time?

What is wrong with my alternative?

Aside from the above mentioned points, a huge number of things:

  1. All the physical constants of nature are tied together. You can't just change one and hope that the others don't notice. If the speed of light is constantly decreasing that has drastic implications throughout all of physics. For example, if the speed of light changes, suddenly the electromagnetic force strength changes, atoms aren't stable, and the universe explodes! What is your proposal for stopping that occurrence?
  2. Just how exactly are things shrinking? Are the atoms getting closing together? Are the atoms themselves shrinking? At some point you're going to hit a limit and your object becomes a black hole. Are you really predicting that eventually the universe will be nothing but black holes? My guess is that if you did such a calculation, you'd find the time it took to reach that point is about a second, but the Universe has been around for a few billion years.
  3. What caused this initial shrinkage? Motion requires energy input. Where did this energy come from? How did it apply itself so uniquely as to cause every individual thing to shrink into itself and only itself?
  4. How did the universe get to be so distributed? Your proposal basically implies the Big Bang didn't happen since you attempt to explain things the Big Bang theory already explains. If your explanation differs and you believe yours is the correct one, then the Big Bang Theory cannot be correct. If that is the case, then how did the universe get into its current state if not the Big Bang?

I'm sure I could go on and on, but hopefully you can see there are more than a few holes in your idea. No offense, but it takes years and years of studying to contribute to the forefronts of science. Ground-breaking ideas aren't going to be had by someone who read about a few concepts from Wikipedia or A Brief History of Time and decided they could come up with a better theory. That's the academic equivalent to reading about the rules of (American) football and deciding you can play in the NFL.

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  • $\begingroup$ Good (and short for the sake of the question) answer. On the other hand can we know light just not "decays"? Lets say any quanta of light redshifts after each billion year traveling in the void. Can we detect that? Any implication other distant galaxies are not accelerating to keep always from us? $\endgroup$
    – jean
    Oct 2, 2017 at 18:46
  • $\begingroup$ @jean There's a difference between "decaying" light and redshifting light. Besides, when light redshifts during travel, that involves an energy loss. The light actually loses energy, as is the case with the cosmological redshift due to the expansion of space. Light just randomly losing energy (by redshifting) as it travels, like you suggest, violates conservation of energy. $\endgroup$
    – zephyr
    Oct 2, 2017 at 20:38
  • $\begingroup$ Sure but I'm just messing around with OP idea of some mechanism to explain redshift in a no expanding universe. With time maybe a post e new demi-nonsense question =) $\endgroup$
    – jean
    Oct 3, 2017 at 10:30
  • $\begingroup$ Awesome answer! Thanks! There's definitely a lot of stuff there that I don't know enough about. So as opposed to the Big Bang, where everything is concentrated at a single point and expands rapidly (have I got that right??). I thought that alternatively, all particles could begin with infinite size, (like a stretched skin of a balloon) and then undergo a giant 'Snap Back' I thought that for an observer inside the universe, those two scenarios might look exactly the same, so I was wondering if there was a chance at those scenarios being equivalent! $\endgroup$
    – Matt
    Oct 3, 2017 at 11:49
  • $\begingroup$ And then since c should be constant relative to the size of particles, that's where the changing c thing came from lol. Thanks guys! $\endgroup$
    – Matt
    Oct 3, 2017 at 11:53
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This hypothesis has far too many contradictions with our current physical models. I'll list a few:

How would you actually know if the galaxies are shrinking, or if they are moving further apart and the view is just zooming out to keep them in frame?

We would see how it agrees with our modern theories. Relativity has matched our measurements with extreme accuracy, so let's just analyze why your hypotheses would go against its basic principles:

"Oh, but the red shift!" I hear people exclaim.

General relativity predicted that our universe was dynamic a decade before the first observations of redshift confirmed it. A static universe is a physical impossibility; anything, from the tiniest thermal fluctuations, would send it perpetually collapsing or expanding.

The speed of light is simply slowing down.

This would violate the second postulate of special relativity, which states the speed of light is always $c$. A massless particle cannot change its speed.

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