So I have read on proofs for the Big Bang theory as it appears to be a bit far-fetched to me (the layman). I mean, definitely there is some genius mathematics behind the fancy yet inaccurate linguistic description, but for me, physics can only be real physics (and not philosophy) if it can be verified experimentally.

One of the strongest observations supporting the BB theory is the red shift or "galaxies appear redder than they should be". I understand this principle as it can easily be compared to the Doppler effect, using light waves instead of sound waves. So if it appears redder than it should be, then indeed I am convinced the galaxies are moving away from us and something like cosmological expansion could be true.

Now my question is, in the phrase "redder than they should be", who determined what the original frequency of the emitted light/radiation of that specific galaxy is? Maybe it is compared with the light our own galaxy emits, but who dares to claim our galaxy is the same as other galaxies in terms of emitted radiation? And then again, I have never seen the observations in raw form (too complex for me). Maybe there are certain "signature" frequencies radiated by familiar processes, that seem slightly "redder", or should I say slightly lower pitched, coming from other galaxies? Could you explain to me what the hard proof is for this assumption?

Then I read on articles providing alternative views on the BB theory (I am skeptic indeed) that reinterpreted certain observations, like microwave background radiation etc. without using the Big Bang or the Fireball itself. In some cases, the articles claimed to reach a more plausible and simpler model without the need for "dark matter" and all of the unobserved, almost controversial concepts. Why are almost all physicist accepting the BB theory, while from my point of view it is still not decently substantiated? It almost seems as if popularized physics by famous physicists talking about the weird, the unfathomable, has monopolized the physics itself. Each time physicists are talking about 10^-32 seconds after the BB I feel exasperated; theories of physics built on theories built on theories built on observations that do not completely match... But then again, I do realize many concepts originate from the mathematics and thus cannot be expressed credibly through words. Yet this red shift issue simply bothers me.

Please correct me if I interpreted the BB theory pertaining to the red shift incorrectly.

  • 1
    $\begingroup$ The term "redder than it should be" sounds a bit weird to me, but we have an extremely solid experimental and theoretical foundation for exactly at which wavelengths the various elements emit light, and how the observed wavelength changes as a function of the emitting object's motion relative to us. Measuring the redshift, and hence velocity, of an object is one of the easiest tasks in astronomy, and can have accuracies down to the order of meters per second, even for an object that are many lightyears away. $\endgroup$
    – pela
    Feb 17, 2015 at 22:47
  • $\begingroup$ @pela I subscribe to the user's question, and I apologize but your answer (quite standard) doesn't convince me. How do we know that billions of years ago the light velocity was the same as today? How do we know that the frequency at which the elements emitted was the same as today? In which base do we assume that his red-shift is due to a movement away of us, and not to a change in the light velocity with respect to the value today? Isn't the light velocity a function of the dimension of the Universe? $\endgroup$
    – Sofia
    Feb 17, 2015 at 23:24
  • $\begingroup$ @Sofia these are valid questions but they do have answers. Pela has given the simplest explanation. You are proposing more complicated explanations that have no evidence for them. For instance we can of course measure the energy level separations in "old atoms" using light from galaxies that are billions of years old. Or, we can tell that G is not changing by precise measurements of solar oscillations; and many other tests. $\endgroup$
    – ProfRob
    Feb 17, 2015 at 23:42
  • $\begingroup$ @RobJeffries please see, I know that Asher Peres (who was the student of Nathan Rosen, former assistant of Einstein) found that the light velocity probably changed as the universe evolved. It appeared in a work of Peres with Daniel Terno, Peres' student for Ph. D. (Now, Danny Terno works at the Perimeter Institute.) I am not able to read that article as I don't work in gravity. But I know their conclusion. This is what I know. $\endgroup$
    – Sofia
    Feb 18, 2015 at 0:24
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    $\begingroup$ I can only agree with @RobJeffries: It's perfectly sane to question the invariance of the speed of light, and it's important that we investigate the validity of this. But as long as we are more convinced of its invariability than of its variability, there is no reason to make any assumptions about the latter. Should it turn out that c does indeed vary in time, it must be very little, lest it would be easy to measure from its other effects (the value of c enters many physical relations). But in order to explain the redshift of light from galaxies, it would have to be changing by a huge factor. $\endgroup$
    – pela
    Feb 18, 2015 at 7:36

1 Answer 1


I'm not dealing with most of this ; the reason most physicists are convinced that something like the big bang model explains many aspects of the universe is that the model has been verified against many observational tests. These are not just limited to the expansion of the universe and predicted properties of the microwave background.

In answer to a couple of specific points. We know that the light from galaxies is red shifted because their spectra contain absorption and emission lines that are characteristic of energy level differences in the atoms and ions that make them up. These characteristic lines can in most cases be easily generated in the laboratory and therefore the whole pattern is seen to be red shifted against laboratory standards.

Furthermore, the redshifts can be demonstrated to be due to the expansion, because we can observe that certain standard cosmic events, like the explosion of a type I supernova, take longer. The time dilation exactly corresponds to their redshifts.

I share your exasperation that many science popularisers imply (sometimes by omission) that we are much more certain about what happened in the first fraction of a second. This is misleading. Most physicists would agree that about 13.7 billion years ago the universe was much smaller than it is now and very hot. The details of anything prior to the first millionth of a second or so are highly obscure and keenly debated.

  • $\begingroup$ Good point with the time dilation matching the redshift. Concerning the first fractions of a second, every time we build a better accelerator, we can reproduce conditions that are closer to t = 0 than previously, and it turns out that results are in good agreement with theory. I think we're down at t ~ 1e-12 s at the moment, but I don't know much about particle physics. $\endgroup$
    – pela
    Feb 18, 2015 at 7:44
  • $\begingroup$ Oke, this did convince me. Absorption and emission spectra are shifted slightly, suggesting strongly that indeed the galaxies are moving away from us. Then again, couldn't this be explained if we assume that the galaxies orbit some point in space, and that our galaxy is lagging behind due to lower angular velocity or something similar? Then we should be able to detect a shift as well. Then again, if we detect red shift in all directions, it could indeed be cosmic expansion. $\endgroup$
    – user209347
    Feb 18, 2015 at 15:24
  • $\begingroup$ @user209347 Yes, the isotropy of redshifts argues against any expansion-from-a-point or orbiting-a-point type scenarios. $\endgroup$
    – ProfRob
    Feb 22, 2015 at 20:07

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