Question: Why is the big bang theory so widely accepted and are there any alternative theories?

Susan Cartwright answered on 14 Jun 2015:

The big bang theory is widely accepted because there is strong evidence in its favour, and because it arises naturally from the General Theory of Relativity, which is itself a well-tested theory. There have been several alternative models since the Big Bang was proposed in the late 1940s, but most of them failed observational tests. There is one currently supported alternative theory, which is more difficult to test, but relevant evidence should be forthcoming in the next few years.
What is the evidence for the big bang? In order to establish this, we need to know what the Big Bang model predicts. Basically, we have the following arguments:
1. In the Big Bang model, the universe is expanding – that is, the spaces between galaxies get larger over time. Incidentally, this is NOT the galaxies rushing away from each other through space: it is the expansion of space itself, rather in the way that spots on a spotted balloon get further away from each other as you blow up the balloon. This expansion was firmly established by Edwin Hubble and Milton Humason in 1931 (after an earlier but much less precise determination by Hubble in 1929), based on their distance measurements combined with spectroscopic evidence that had been accumulating since 1915 (it is often suggested that this was an unexpected discovery by Hubble, but that is not the case). This counts as a successful prediction, because the equations that describe the Big Bang expansion had first been written down by Aleksandr Friedman in 1922, and later rediscovered by Georges Lemaitre in 1927. (Hubble does not seem to have known about these predictions when he wrote his papers; he does not mention them.) However, it is not a unique prediction: other models, such as the Steady State model of Hermann Bondi, Tommy Gold and Fred Hoyle, also predict expansion. So we can’t really count it as strong evidence in favour of the Big Bang theory.
2. In the Big Bang model, the universe has a finite age – if we run time backwards, everything becomes closer and closer together, and hotter and hotter, until you arrive at the beginning of time when the universe was (at least in the original version of the theory) infinitely hot and dense. Because light does not travel infinitely fast, when you look at distant galaxies you are seeing them as they were a long time ago. Therefore, the Big Bang predicts that distant galaxies should look much younger than the nearby galaxies we see around us. This turns out to be true: distant galaxies do look different from nearby ones, and some of those differences suggest that they are younger. The most obvious difference is that far more distant galaxies have active nuclei – that is, their central regions are emitting vast amounts of energy at all sorts of wavelengths – radio waves, X-rays, infra-red. This is probably caused by matter falling on to an extremely massive black hole (about 100 million or more times the mass of the Sun). The point is that we can calculate how long this kind of extreme activity is likely to last, and the result is much less than the age of our own Galaxy, or indeed our own Sun. So it is to be expected that such activity would be much more common when the galaxies were all much younger, and this is a good explanation of why we now see it as much more common in galaxies that are very distant (and so seen as they were billions of years ago).
This genuinely is evidence in favour of the Big Bang, because the most popular alternative model of the 1950s – the Steady State – held that although the universe was expanding, it was nevertheless infinitely old (new matter being continuously created to balance the expansion and keep the number of galaxies per billion cubic light years constant). The Steady State would have expected that galaxies would appear to have the same average age at any time, and thus at any distance – so there should not be more active galactic nuclei at large distance. This observation, which was first made in 1963, is therefore evidence against the Steady State.
3. The Big Bang predicts that the early universe was very hot and very dense. This is because as you run time backward, the galaxies get closer and closer together, so eventually they all merge into one big lump. Therefore the early universe in the Big Bang model should be very dense. This was realised already in the 1920s: Georges Lemaitre talked about “the primeval atom”. Also, when something expands it usually cools down, because you are spreading energy over a greater volume, so the early universe should be very hot – this wasn’t really pointed out until the work of George Gamow and his students in the late 1940s and early 1950s.
This has two consequences. The first is that about 3 minutes after the Big Bang, the temperature of the universe is comparable to the central cores of stars, where nuclear fusion reactions take place. Therefore, the free protons and neutrons that were formed even earlier can combine to make the first few nuclei of the periodic table: deuterium (the heavy isotope of hydrogen, with one neutron bound to one proton), the two isotopes of helium (He-3, with one neutron and two protons, and the much more common He-4, with two of each), and a little bit of lithium-7 (3 protons and 4 neutrons). Furthermore, the nuclear physics reactions contributing to this have been studied in the laboratory, and we know how they work, so we can calculate how much of each isotope should be made. It turns out that the amount of deuterium, in particular, is a very good test of the Big Bang, because deuterium is not made anywhere else: stars actually destroy deuterium, converting it to helium-4. The amount of deuterium we observe is as the Big Bang calculations predict, and it is consistent with the amount of He-4 we observe (He-4 _is_ made in stars, but the amount that we see in the universe is far too much to have been made by all the stars that have ever lived: most of it was made in the early universe). This is very good evidence for the Big Bang, because not only does deuterium exist in the universe, it exists at exactly the level predicted by the Big Bang calculation.
The second effect of the hot, dense early universe is that hot, dense materials give off light (think of a red-hot poker, a glowing coal, molten metal, an old-fashioned tungsten-filament electric light). What is more, the light that they give off is very precisely understood: it follows a law called the Planck function (first worked out by Max Planck in 1901), which depends only on the temperature of the material. Therefore, if the early universe was hot and dense, there should be left-over light from that period, and it should obey the Planck function – although, because of the cooling effect of expansion, its temperature should now be only a few degrees above absolute zero. This low temperature puts it in the microwave region of the spectrum, with wavelengths of millimetres (unlike visible light, which has wavelengths of tenths of a micrometre). This radiation was predicted by Gamow’s group in 1950, but nobody in 1950 knew how to detect these wavelengths, and the prediction was forgotten. It was rediscovered in 1965 by a group led by Bob Dicke at Princeton, and the same year it was observed – accidentally – by Penzias and Wilson. By 1968, it has been observed at enough wavelengths to show that it followed the Planck function, as far as could be checked (we now know that it follows the Planck function very precisely indeed). This was compelling evidence for the Big Bang – no other theory (then or now) could explain why the universe should contain a background radiation that obeyed the Planck function for a temperature a few degrees (2.725, to be precise) above absolute zero. The Big Bang has been the standard theory of cosmology ever since, and nobody has come up with any evidence to contradict this.

The modern alternatives to the Big Bang accept pretty much all the evidence above. The main area of disagreement is what happened in the very early stages, long before even the formation of elements at 3 minutes. The standard description of this since 1981 has been a theory called “inflation”, which its inventor Alan Guth calls “a prequel to the Big Bang.” Inflation holds that a tiny tiny fraction of a second after the beginning, there was a brief period of enormously rapid expansion, much much faster than the speed of light (don’t panic – nothing can move THROUGH space faster than the speed of light, but space itself can expand as fast as it wants), and that our entire visible universe sprang from an incredibly small piece of pre-inflation space, about 1/100-billion times the diameter of an atomic nucleus. This explains why the universe we see is so uniform in all directions, which was a puzzle in the original big bang model. However, some theorists think that instead, the stage for the Big Bang was set by a collision between entire universes. These theorists believe that there are more than 3 space dimensions, and that our 3-dimensional universe is moving in a 4th space dimension and periodically collides with another 3-dimensional universe. The collision wipes out all the features of the universe and looks to later observers just like a Big Bang (which I suppose it literally was, but not in the way intended in the standard Big Bang!).
It is hard to tell the difference between the Big Bang + inflation model and the extra-dimensional collision model, because after the first tiny fraction of a second they basically look exactly the same. The possible solution is that inflation predicts faint ripples in the fabric of spacetime – “primordial gravitational waves” – which leave faint swirling imprints in the microwave background. You may remember that there was a big news story about a year ago, when a telescope called BICEP2 claimed to have found evidence for this swirling pattern. It turns out that they were probably wrong – they had underestimated the effects of dust in the Milky Way Galaxy – but more sensitive studies of this effect are being carried out. The extra dimensions model does not predict these patterns, so if they are found it will be evidence against extra dimensions and in favour of inflation.

I’m sorry this was such a long answer, but it is an important question. Many people seen to think that the Big Bang is just a guess, and that scientists believe it because it’s in the textbooks. I hope that I’ve convinced you that this is not the case: the Big Bang is a valid scientific theory, and like all valid scientific theories it is still around after nearly 100 years because it has stood up to experimental test.