The Universe is Shrinking

Dark energy is one of the mysteries of modern science. It is supposed to amount to about 68% of the Universe, yet we have no idea what it is. Its discovery led to Nobel prizes, yet it is now considered possible that it does not even exist. To add or subtract 68% of the Universe seems a little excessive.

One of the early papers (Astrophys. J., 517, pp565-586) supported the concept. What they did was to assume type 1A supernovae always gave out the same light so by measuring the intensity of that light and comparing it with the red shift of the light, which indicates how fast it is going away, they could assess whether the rate of expansion of the universe was even over time. The standard theory at the time was that it was, and it was expanding at a rate given by the Hubble constant (named after Edwin Hubble, who first proposed this). What they did was to examine 42 type 1a supernovae with red shifts between 0.18 and 0.83, and compared their results on a graph with what they expected from the line drawn using the Hubble constant, which is what you expect with zero acceleration, i.e. uniform expansion. Their results at a distance were uniformly above the line, and while there were significant error bars, because instruments were being operated at their extremes, the result looked unambiguous. The far distant ones were going away faster than expected from the nearer ones, and that could only arise if the rate of expansion were accelerating.

For me, there was one fly in the ointment, so to speak. The value of the Hubble constant they used was 63 km/s/Mpc. The modern value is more like 68 or 72; there are two values, and they depend on how you measure them, but both are somewhat larger than this. Now it follows that if you have the speed wrong when you predict how far it travelled, it follows that the further away it is, the bigger the error, which means you think it has speeded up.

Over the last few years there have been questions as to exactly how accurate this determination of acceleration really is. There has been a question (arXiv:1912.04903) that the luminosity of these has evolved as the Universe ages, which has the effect that measuring the distance this way leads to overestimation of the distance. Different work (Milne et al. 2015.  Astrophys. J. 803: 20) showed that there are at least two classes of 1A supernovae, blue and red, and they have different ejecta velocities, and if the usual techniques are used the light intensity of the red ones will be underestimated, which makes them seem further away than they are.

My personal view is there could be a further problem. The type 1A occurs when a large star comes close to another star and begins stripping it of its mass until it gets big enough to ignite the supernova. That is why they are believed to have the same brightness: they ignite their explosion at the same mass so there are the same conditions, so there should be the same brightness. However, this is not necessarily the case because the outer layer, which generates the light we see, comes from the non-exploding star, and will absorb and re-emit energy from the explosion. Hydrogen and helium are poor radiators, but they will absorb energy. Nevertheless, the brightest light might be expected to come from the heavier elements, and the amount of them increases as the Universe ages and atoms are recycled. That too might lead to the appearance that the more distant ones are further away than expected, which in turn suggests the Universe is accelerating its expansion when it isn’t.

Now, to throw the spanner further into the works, Subir Sarkar has added his voice. He is unusual in that he is both an experimentalist and a theoretician, and he has noted that the 1A supernovae, while taken to be “standard candles”, do not all emit the same amount of light, and according to Sarkar, they vary by up to a factor of ten. Further, previously the fundamental data was not available, but in 1915 it became public. He did a statistical analysis and found that the data supported a cosmic acceleration but only with a statistical significance of three standard deviations, which, according to him, “is not worth getting out of bed for”.

There is a further problem. Apparently the Milky Way is heading off in some direction at 600 km/s, and this rather peculiar flow extends out to about a billion light years, and unfortunately most of the supernovae studied so far are in this region. This drops the statistical significance for cosmic expansion to two standard deviations. He then accuses the previous supporters of this cosmic expansion as confirmation bias: the initial workers chose an unfortunate direction to examine, but the subsequent ones “looked under the same lamppost”.

So, a little under 70% of what some claim is out there might not be. That is ugly. Worse, about 27% is supposed to be dark matter, and suppose that did not exist either, and the only reason we think it is there is because our understanding of gravity is wrong on a large scale? The Universe now shrinks to about 5% of what it was. That must be something of a record for the size of a loss.

How Fast is the Universe Expanding?

In the last post I commented on the fact that the Universe is expanding. That raises the question, how fast is it expanding? At first sight, who cares? If all the other galaxies will be out of sight in so many tens of billions of years, we won’t be around to worry about it. However, it is instructive in another way. Scientists make measurements with very special instruments and what you get are a series of meter readings, or a printout of numbers, and those numbers have implied dimensions. Thus the number you see on your speedometer in your car represents miles per hour or kilometers per hour, depending on where you live. That is understandable, but that is not what is measured. What is usually measured is actually something like the frequency of wheel revolutions. So the revolutions are counted, the change of time is recorded, and the speedometer has some built-in mathematics that gives you what you want to know. Within that calculation is some built-in theory, in this case geometry and an assumption about tyre pressure.

Measuring the rate of expansion of the universe is a bit trickier. What you are trying to measure is the rate of change of distance between galaxies at various distances from you, average them because they have random motion superimposed, and in some cases regular motion if they are in clusters. The velocity at which they are moving apart is simply change of distance divided by change of time. Measuring time is fine but measuring distance is a little more difficult.  You cannot use a ruler.  So some theory has to be imposed.

There are some “simple” techniques, using the red shift as a Doppler shift to obtain velocity, and brightness to measure distance. Thus using different techniques to estimate cosmic distances such as the average brightness of stars in giant elliptical galaxies, type 1a supernovae and one or two other techniques it can be asserted the Universe is expanding at 73.5 + 1.4 kilometers per second for every megaparsec. A megaparsec is about 3.3 million light years, or three billion trillion kilometers.

However, there are alternative means of determining this expansion, such as measured fluctuations in the cosmic microwave background and fluctuations in matter density of the early Universe. If you know what the matter density was then, and know what it is now, it is simple to calculate the rate of expansion, and the answer is, 67.4 +0.5 km/sec/Mpc. Oops. Two routes, both giving highly accurate answers, but well outside any overlap and hence we have two disjoint sets of answers.

So what is the answer? The simplest approach is to use an entirely different method again, and hope this resolves the matter, and the next big hope is the surface brightness of large elliptical galaxies. The idea here is that most of the stars in a galaxy are red dwarfs, and hence the most “light” from a galaxy will be in the infrared. The new James Webb space telescope will be ideal for making these measurements, and in the meantime standards have been obtained from nearby elliptical galaxies at known distances. Do you see a possible problem? All such results also depend on the assumptions inherent in the calculations. First, we have to be sure we actually know the distance accurately to the nearby elliptical galaxies, but much more problematical is the assumption that the luminosity of the ancient galaxies is the same as the local ones. Thus in earlier times, since the metals in stars came from supernovae, the very earliest stars will have much less so their “colour” from their outer envelopes may be different. Also, because the very earliest stars formed from denser gas, maybe the ratio of sizes of the red dwarfs will be different. There are many traps. Accordingly, the main reason for the discrepancy is that the theory used is slightly wrong somewhere along the chain of reasoning. Another possibility is the estimates of the possible errors are overly optimistic. Who knows, and to some extent you may say it does not matter. However, the message from this is that we have to be careful with scientific claims. Always try to unravel the reasoning. The more the explanation relies on mathematics and the less is explained conceptually, the greater the risk that whoever is presenting the story does not understands it either.