Dismantling the belief in a static universe, Edwin Hubble's revolutionary observations in the 1920s laid the groundwork for our understanding of a continually expanding cosmos. However, we must seek to reconcile this theory with observations that are consistent with a non-expanding universe, writes Tim Anderson.
You have been taught that the universe began with a Big Bang, a hot, dense period about 13.8 billion years ago. And the reason we believe this to be true is because the universe is expanding and, therefore, was smaller in the past. The Cosmic Microwave Background is the smoking gun for the Big Bang, the result of a reionization of matter that made the universe transparent about 300–400,000 years after the Big Bang.
How did we go from Einstein modifying his equations to keep the universe static and eternal, which he called the biggest blunder of his life, to every scientist believing that the universe had a beginning in 10 years? It all started with astronomer Edwin Hubble using the most powerful telescope at the time on Mount Wilson in California. At the time, in the 1920s, scientists believed that the Milky Way galaxy was the totality of the universe. Objects in the night sky like Andromeda that we now know are galaxies were called “nebulae”.
Looking at these objects, however, Hubble knew how bright particular stars called Cepheid variables were supposed to be. Knowing how bright they were supposed to be meant that he could tell how far away they were. He found to his surprise that Andromeda and Triangulum had Cepheid variables that were too far away to be inside the Milky Way. They weren’t nebulae. They were galaxies.
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At the time, in the 1920s, scientists believed that the Milky Way galaxy was the totality of the universe.
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Hubble’s discoveries, made in 1924, merited a short column on page 6 of the New York Times. In that article, “Dr. Hubbell” was said to have shown that nebulae are in fact “island universes”. The concept was so new that they weren’t even recognized as galaxies. Hubble was able to estimate distances for his newly discovered galaxies. His estimates were off by about a factor of 7 but proportionally correct. Other scientists such as Vesto M. Slipher, had been busy, since 1912, measuring how fast the galaxies he identified were moving towards or away from us by measuring their redshift.
The way you measure redshift uses a concept from atomic theory called spectroscopy. Basically, stars contain elements that absorb light at specific wavelengths. These are patterns of missing wavelengths in the spectrum of the light called absorption spectra. These patterns show up because the atoms contain electrons that absorb photons with particular frequencies. When the photon strikes the atom, the electron absorbs it and moves to a higher orbital, but only if it has the exact frequency needed for that electron. Otherwise, no absorption happens. This property can be used to determine what things are made of by exposing them to light and measuring their emissions. It can also be used to make lasers.
Spectrum of the star Altair from NASA, ESA, Leah Hustak (STScI).
In astronomy, it is how we determine how fast objects are moving towards or away from us because of something called the Doppler effect. If something is moving away from us, the wavelengths of light coming from that object will be stretched out which makes them longer and lower frequency. This shifts the absorption spectrum to the right in the above picture and so is called redshift since the right side is red. If the object is moving towards us, then it will be shifted to the blue side and so is called blueshift. The same thing happens with sound which is why a siren has a higher pitch as an ambulance moves towards you and a lower pitch when it moves away from you.
Since we know what the frequencies in the absorption spectrum are supposed to be for particular elements and we can, by the pattern and what we know about stars, identify what those elements should be. We can determine how redshifted stars and galaxies are. When Hubble looked at all these new galaxies he had identified, he made a correlation between their velocity based on redshift and their distance based on the Cepheid variables. It turns out that these were linearly correlated. In other words, the further away a galaxy was, the faster it moved away from us. You can make a graph with speed on the vertical axis in km/s and distance on the horizontal axis in Megaparsecs (about 3.26 million lightyears) and you will find that it makes a line.
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In an expanding universe, the expansion itself decreases the surface brightness of galaxies and their size, so galaxies should be dimmer and smaller in an expanding universe than in a non-expanding one.
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Hubble identified the slope of this line as a universal constant which we now know as the Hubble constant. His value was about 500 km/s/Megaparsec. If you correct for his factor of 7 error in distance, this falls within the currently accepted value of 68–74 km/s/Megaparsec. Alexander Friedmann in 1922 and Fr. George Lemaître independently in 1927 had used Einstein’s field equation to predict that the universe should be expanding (or shrinking). Combining their results with Hubble’s observations and the successful demonstration of the correctness of Einstein’s equations within the Solar System, scientists concluded that the universe was expanding.
Not everyone was happy about this conclusion. That included Hubble himself. Hubble disagreed with the interpretation of his data believing that redshifts might not be related to velocity at all and he criticised the popularity of the expanding universe theory, saying in the Journal of the Royal Astronomical Society in 1937:
The interpretation of red shifts by the theory of the expanding universes is so plausible and so widely current that, in making a delicate test of the theory, it is desirable to push uncertainties in the favourable direction before admitting a discordance.
He had good reason to believe in a discordance because, based on his data, the universe would have been younger than the Earth, too small and dense by far, with a “closed” geometry implying it should fall back in on itself. This turned out to be wrong because Hubble had vastly underestimated the distances to the galaxies he had observed. The universe was actually far older and less dense than he believed.
We now know that universe’s expansion rate appears to give it a flat geometry, neither “open” nor “closed”, a suspiciously finely tuned result explained by a precise amount of dark energy that is only about one order of magnitude higher than the amount of ordinary matter. We now do a similar analysis to what Hubble did, but, instead of Cepheid variables, we use type Ia Supernovae which also have a standard brightness so we can estimate their distance independently of redshift.
Redshift is not only a stand in for a galaxy’s distance from us, but, because of how long it takes for light to reach us, a stand in for how far back in the past we are looking when we observe that galaxy. If we look as far back as possible, back to the most redshifted light we can see, all we see is a bright, almost uniform glow: the Cosmic Microwave Background, and that is the earliest, most distant light we can see. This light is so redshifted it appears at similar frequencies to the radio waves we use to cook things in microwave ovens. You can’t use the CMB to cook things, unfortunately, unless those things are cooler than about 2.7 degrees above absolute zero.
Since Hubble made his objections, other evidence for the expanding universe has been discovered. We see that the universe appears to be denser in the past, which is correlated with looking further away. We also see time dilation, a relativistic effect, in type 1a Supernovae that is consistent with an expanding universe. From the theoretical perspective, also, if Einstein’s equations are true, a static universe would be unstable.
Why then even question the theory after all this time? Well, because not everything perfectly agrees with it.
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Unlike distant stars which are compact and have a magnitude, for objects that are extended in our telescopes, such as galaxies, they have a surface brightness. That surface brightness is basically the sum total of the brightness of luminous bodies within them minus all the dust between us absorbing that brightness. That goes for anywhere in the electromagnetic spectrum from radio waves on up. That is only true, however, in a universe that is not expanding.
In an expanding universe, the expansion itself decreases the surface brightness of galaxies and their size, so galaxies should be dimmer and smaller in an expanding universe than in a non-expanding one. What’s more, in an expanding universe, the angular diameter of a galaxy should be smaller than the surface brightness by an extra factor of (1+z) where z is the redshift of the galaxy. This is called the distance duality relation.
This relation has been a thorn in the expanding universe theory’s side for a while because what we have generally found is that, given some reasonable, general assumptions about how big and bright galaxies are supposed to be over time in the history of our universe, like, assuming they are more or less the same through time, the observations are consistent with a non-expanding universe. That goes for both the UV spectrum and the radio spectrum.
In an expanding universe, the angular size of a galaxy should shrink by the redshift to the fourth power whereas, in a non-expanding one, it should just shrink linearly with redshift. What do we see? We see that angular sizes shrink linearly! In fact, if the sizes obeyed the fourth power law, we would hardly be able to see distant galaxies at all. The only way to reconcile these results is to assume that the actual brightness and size of galaxies evolves in a finely tuned way to agree with the expanding universe theory, which is deeply unsatisfying.
What’s more, ancient galaxies would have to have higher luminosity density (how bright each patch of them is) to the tune of a cube of the redshift! This is especially true of galaxies that are closer to us in time, within the last 5 billion years or so. The truth is that if the universe is expanding, we would expect that the distance between us and a galaxy to be increasing the whole time that the light is traveling from it to us. The light travels farther and so that would make the galaxy seem smaller as if the galaxy were further away. That means that the relationship between brightness and size should be different as well.
The main way to reconcile this problem is to point it out as an example of fine tuning. Galaxies were actually considerably brighter in the past and that is proportional to how long ago they existed. In other words, galaxies have been getting dimmer and smaller as the universe has gotten older in a way that is finely tuned relative to expansion to make it appear that they match a non-expanding universe.
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The truth is that if the universe is expanding, we would expect that the distance between us and a galaxy to be increasing the whole time that the light is traveling from it to us.
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The alternative is that the universe is not expanding, but if that’s the case then we would have to explain all the observations that appear to support that expansion: the apparent Hubble constant, the predictions of Einstein’s equations, time dilation of type 1a Supernovae, and the Cosmic Microwave Background (both now and in the past when it was hotter).
Is there any other theory of a non-expanding universe that could explain all of that? Well, what have we got? Both the current model of the universe and the alternative which has no beginning, the Steady State theory, are both expanding universe models. The only difference is the Steady State model creates matter continuously rather than having a fixed origin in the past. So we do away with those.
Then there’s tired light. Tired light just says that light loses energy as it interacts with matter fields as it travels through space. That explains redshift but it doesn’t explain much else. It doesn’t explain the sharpness of how distant galaxies appear, for example, which suggests the light hasn’t had a lot of interaction on the way to us. It doesn’t explain time dilation either, which matches an expanding universe model. It also doesn’t explain the Cosmic Microwave Background. The CMB isn’t just some background light of the universe. It has a very precise temperature curve, called a blackbody curve.
In the tired light model, the universe hasn’t changed size so the density of light would be the same now as when the CMB was emitted, the light is just more redshifted. That means that the density of light hasn’t changed whereas, in an expanding universe model, the density of light has decreased in proportion to the redshift.
We can see this density relative to redshift in the temperature spectrum of the CMB. As the density decreases, the CMB cools. In order to account for this, in a tired light model, the CMB would have to have very small redshift so that its redshift hasn’t increased that much relative to density. Observations (from, e.g., FIRAS) would then put the tired light CMB as originating 0.25 Megaparsecs or less away which is closer than Andromeda! We know it can’t have come from there because the universe is transparent beyond Andromeda. The CMB has to come from somewhere that isn’t transparent, which must be very far away.
So the tired light model doesn’t work. It is dead. The only other theory is called the intrinsic redshift theory which is a vague hand waving idea that redshift is explained by “something else” that is intrinsic to distant objects. I think that is a much bigger leap than just assuming galaxies were mysteriously brighter and bigger in the past in a relationship with redshift.
The fact is that there are several mysterious fine-tuning relationships in cosmology and so adding one more to the pile is a much easier leap than trying to abandon an otherwise successful theory. Fine tunings tend to indicate that things are related in ways that we don’t understand, so the goal is to increase our understanding of those things, i.e., how galaxies evolved in the early universe and how that evolution may be related to luminosity and might give the redshift relationship.
And that is a far better direction that trying to resurrect theories for non-expanding universes. They may fit some of our philosophies better, but they aren’t consistent with the universe that we can see.
References
Li, Pengfei. “Distance Duality Test: The Evolution of Radio Sources Mimics a Nonexpanding Universe.” The Astrophysical Journal Letters 950.2 (2023): L14.
Lerner, Eric J., Renato Falomo, and Riccardo Scarpa. “UV surface brightness of galaxies from the local universe to z~ 5.” International Journal of Modern Physics D 23.06 (2014): 1450058.
Lerner, Eric J. “Observations contradict galaxy size and surface brightness predictions that are based on the expanding universe hypothesis.” Monthly Notices of the Royal Astronomical Society 477.3 (2018): 3185–3196.
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