How do we know its existence? Various observations suggest it, such as certain gravitational effects which cannot be explained by visible objects. Galaxies spin, but some spin as if they contain much more matter than is visible. Imagine trying to spin a stone on the end of a string, but there is an invisible rock hanging in the middle of the string. The stars emit light, and this light sometimes seems to âbendâ. Thanks to Albert Einstein, we know this phenomenon well. It is caused by objects whose gravity significantly changes the course of light, just as if the objects act like lenses. But from some stars, we see the light bending, so we know that there is a massive object between them and us, but we cannot see it or detect it. In fact, some estimates suggest that up to 85% of the mass of the substance that fills our universe is dark matter. No prank on an unsuspecting woman added to her weight on this scale. But it was for the discovery of this dark matter (strictly “dark energy”, but I’ll skip that distinction here) that Saul Perlmutter, Adam Riess, and Brian Schmidt won the Nobel Prize in Physics in 2011.
Estimates like these come to us from studying the very early growth of the universe. Cosmologists have collected data on microwaves that left their sources billions of years ago, when the universe was only 380,000 years old. Today, 380,000 may seem like a huge number, but compared to the age of the universe today – around 14 billion years – that’s a very small slice of time. By cosmological standards, the universe was then extremely young. So if the light from such sources tells us anything, it is about the state of the universe at this very young age. And what he tells us is the same story as above: The objects we can see or detect are only a small fraction of the cosmos. But by observing this “cosmic microwave background” (CMB) radiation, astronomers get a map of the universe at this young moment. They can then, in a sense, play the tape of the evolution of the universe from that time to today, as it follows Einstein’s Theory of Gravity. This theory of the cosmos was born from the discovery of dark matter and is called ÃMDP. (This is the Greek letter lambda,Ã, meaning dark energy, and Cold Dark Matter, CDM.)
Reading the tape in this way gives us an idea of ââhow fast the universe is expanding. Specifically, how quickly the expansion of the universe is accelerating: and this measure is the Hubble constant, which I mentioned in my last column here. By this theory and method, the Hubble constant is equivalent to approximately 67.4 kilometers per second per megaparsec (km / s / Mpc). Since the Planck Space Telescope was originally used to map the CMB, this is generally referred to as the Planck estimate of the Hubble constant. Other observations of the young universe produced the same figure.
Why is all this important? Why is dark matter, well, important?
Well, at a famous meeting in cosmological circles – in Santa Barbara in 2019 – Adam Riess himself gave a talk in which he offered evidence of something that had irritated these circles for several years. The universe, he said, was expanding faster than the accepted value of the Hubble constant would suggest.
A group of astronomers calling themselves HoLiCOW (H0 lenses in the COSMOGRAIL well – no matter the expansion except to note the reference to a lens, and H0 is the Hubble constant) – worked with a rather different method of measuring the expansion of the universe. They use quasars whose light has been distorted – “gravitational lens” – by massive galaxies on its way to us. The effect of this lens is that we see a quasar not only as a point of light, but as several bright images. This means that the light has followed different paths around the intermediate galaxy reaching us at different times. It is particularly relevant and useful here that the quasars vary in brightness. The HoLiCOW team timed this variation in each lens image, which gave them the delay between the different light paths, which is directly related to the Hubble constant.
HoLiCOW used six quasars to collect this data. Their measure of the constant matches well with the value that astronomers got using Gaia and the Hubble Space Telescope (HST), which my last column explained. This value is approximately 73.3 km / s / Mpc.
You’ve probably noticed the difference. HoLiCOW has a number for the constant which is almost 9% greater than ÃCDM number. Riess and his colleagues, in turn, worked with Gaia and the HST and yet another method – a “cosmic distance scale,” meaning they walked the universe step by step to calculate distances. Their estimate of the Hubble constant was 74 km / s / Mps – as you can see, in agreement with HoLiCOW.
Is it just a measurement error? No, because this value for the constant appeared again and again as colleagues at HoLiCOW and Riess worked to refine their estimates. In fact, this is a statistically significant difference from Planck’s number – a âfive sigmaâ difference. It’s real enough that scientists have even given it a name, Hubble’s ‘strain’. Some are starting to speculate that there is something else going on here, something fundamental that we haven’t figured out yet, something that is missing in the ÃCDM model of the universe that cosmologists have built over the years. This something acts to accelerate the expansion of the universe.
At that Santa Barbara meeting, Riess put it this way: âIf the recent and early universes disagree, we must be open to the possibility of new physics. Yet the true measure of this tension may be that there are others. At that same gathering in Santa Barbara, other astronomers weren’t quite ready to follow Riess’s line of thought. Wendy Freedman and Barry Madore, wife and husband at the University of Chicago, used different stars for their calculations. They concluded that the Hubble Constant was 69.8 km / s / Mpc. That’s well below Riess’ 74, not much above Planck’s 67.4 number, certainly not a “five sigma” gap. A few months later, they had further refined their estimate, to 69.6 km / s / Mpc.
So what is going on? The short answer is really “we don’t know”. There is clearly a gap that needs to be taken into account. But is it due to dark matter? Or a new physics? What?
There are rumors that in Santa Barbara, Madore sang these lines from an old song: “Clowns on my left, pranksters on the right; here I am stuck in the middle with you!“
To which I might add, “HoLiCOW!”
Formerly a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun
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