Rotation, Rotation

You have probably heard of dark matter. It is the stuff that is supposed to be the predominant matter of the Universe, but nobody has ever managed to find any, which is a little embarrassing when there is supposed to be something like about 6 times more dark matter in the Universe than ordinary matter. Even more embarrassing is the fact that nobody has any real idea what it could be. Every time someone postulates what it is, and work out a way to detect it, they find absolutely nothing. On the other hand there may be a simpler reason for this. Just maybe they postulated what they thought they could find, as opposed to what it is, in other words it was a proposal to get more funds with the uncovering the nature of the Universe as a hoped-for by-product.

The first reason why there might be dark matter came from the rotation of galaxies. Newtonian mechanics makes some specific predictions. Very specifically, the periodic time for an object orbiting the centre of mass at a distance r varies as r^1.5. That means that say there are two orbiting objects, say Earth and Mars, where Mars is about 1.52 times more distant, the Martian year is about 1.88 Earth years. The relationship works very well in our solar system, and it was from the unexpected effects on Uranus that Neptune was predicted, and found to be in the expected place. However, when we take this up to galactic level, things come unstuck. As we move out from the centre, stars move faster than predicted from the speed of those in the centre. This is quite unambiguous, and has been found in many galaxies. The conventional explanation is that enormous quantities of cold dark matter provide the additional gravitational binding.

However, that explanation also has problems. A study of 175 galaxies showed that the radial acceleration at different distances correlated with the amount of visible matter attracting it, but the relationship does not match Newtonian dynamics. If the discrepancies are due to dark matter, one might expect the dark matter to be present in different amounts in different galaxies, and different parts of the same galaxy. Any such relationship should have a lot of scatter, but it hasn’t. Of course, that might be a result of dark matter being attracted to ordinary matter.

There is an alternative explanation called MOND, which stands for modified Newtonian gravity, which proposes that at large distances and small accelerations, gravity decays more slowly than the inverse square law. The correlation of the radial acceleration with the amount of visible matter would be required by something like MOND, so that is a big plus for it, although the only reason it was postulated in this form was to account for what we see. However, a further study has shown there is no simple scale factor. What this means is that if MOBD is correct the effects on different galaxies should be essentially dependent on the mass of visible matter but it isn’t. MOND can explain any galaxy, but the results don’t translate to other galaxies in any simple way. This should rule out MOND without amending the underlying dynamics, in other words, altering Newtonian laws of motion as well as gravity. This may be no problem for dark matter, as different distributions would give different effects. But wait: in the previous paragraph it was claimed there was no scatter.

The net result: there are two sides to this: one says MOND is ruled out and the other says no it isn’t, and the problem is that it is observational uncertainties that suggest it might be. The two sides of the argument seem to be either using different data or are interpreting the same data differently. I am no wiser.

Astronomers have also observed one of the most distant galaxies ever, MACS1149-JD1, which is over ten billion light years away, and it too is rotating, although the rotational velocity is much slower than galaxies that we see that are much closer and nowhere near as old. So why is it slower? Possible reasons include it has much less mass, hence the gravity is weaker.

However, this galaxy is of significant interest because its age makes it one of the earliest galaxies to form. It also has stars in it estimated to be 300 million years old, which puts the star formation at just 270 million years after the Big Bang. The problem with that is it is in the dark period, when matter as we know it had presumably not formed, so how did a collection of stars start? For gravity to cause a star to accrete, it has to give off radiation but supposedly no radiation was given off then. Again, something seems to be wrong. That most of the stars are just this age makes it appear that the galaxy formed about the same time as the stars, or put it another way, something made a whole lot of stars form at the same time in places where the net result was a galaxy. How did that happen? And where did the angular momentum come from? Then again, did it happen? This is at the limit of observational techniques, so have we drawn a non-valid conclusion from difficult to interpret data. Again, I have no idea, but I mention this to show there is a still a lot to learn about how things started.


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.