How do we know of its existence? Various observations suggest it—like certain gravitational effects that are not explained by visible objects. Galaxies spin, but some spin as if they contain much more matter than is visible. Imagine trying to whirl a stone on the end of a string, but there’s an invisible boulder hanging from the middle of the string. Stars emit light, and this light sometimes seems to “bend”. Thanks to Albert Einstein, we know this phenomenon well. It is caused by objects whose gravity noticeably changes the course of light—exactly as if the objects are acting like lenses. But from some stars, we see the light bending, so we know there’s a massive object between them and us, but we can’t see or otherwise detect it. In fact, some estimates suggest that up to 85% of the mass of the stuff that fills our universe is dark matter. No prank on an unsuspecting wife has added to her weight on that scale. But it was for the discovery of this dark matter (strictly “dark energy”, but I’ll gloss over 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 collected data about microwaves that left their sources many billion years ago, when the universe was just 380,000 years old. Now 380,000 may seem like an enormous number, but compared to the age of the universe today—about 14 billion years—it is a vanishingly tiny sliver of time. By cosmological standards, the universe was extremely young then. So if the light from such sources tells us something, it is about the state of the universe at that very young age. And what it tells us is the same story as above: the objects we can see or detect make up only a small fraction of the cosmos. But by observing this “cosmic microwave background” (CMB) radiation, astronomers get a map of the universe at that young moment. They can then, in a sense, play the tape of the universe’s evolution from that time to today, as it follows Einstein’s theory of gravity. This theory of the cosmos grew out of the discovery of dark matter, and is called ÙCDM. (That’s the Greek letter lambda,Ù, standing for dark energy, and Cold Dark Matter, CDM.)
Playing the tape in this fashion gives us an idea how fast the universe is expanding. More accurately, how much the expansion of the universe is accelerating: and that measure is the Hubble Constant, which I mentioned in my last column here. By this theory and method, the Hubble Constant works out to about 67.4 kilometers per second per megaparsec (km/s/Mpc). Since the Planck Space Telescope was originally used to map the CMB, this is usually referred to as the Planck estimate of the Hubble Constant. Other observations of the young universe have produced the same figure.
Why does all this matter? Why does dark matter, well, matter?
Well, at a meeting famous in cosmological circles— in Santa Barbara in 2019— Adam Riess himself gave a lecture in which he offered evidence for something that had rankled in those circles for several years. The universe, he said, was expanding faster than that accepted value of the Hubble Constant would suggest.
A group of astronomers calling themselves HoLiCOW (H0 Lenses in COSMOGRAIL’s Wellspring—never mind the expansion except to note the reference to a lens, and H0 is the Hubble Constant)— has been working with a quite different method to measure the expansion of the universe. They use quasars whose light has been bent— “gravitationally lensed”— by massive galaxies on its way to us. The effect of this lensing is that we see a quasar not just as one spot of light, but as several bright images. This means that the light has followed different paths around the intervening galaxy to reach us at different times. Of particular relevance and utility here is that quasars vary in brightness. The HoLiCOW team timed this variation in each lensed image, which gave them the delay between the different paths of light. This delay is related directly to the Hubble Constant.
HoLiCOW used six quasars to gather this data. Their measurement of the Constant agrees well with the value astronomers have got using Gaia and the Hubble Space Telescope (HST), which my last column explained. That value is about 73.3km/s/Mpc.
No doubt you noticed the difference. HoLiCOW has a number for the Constant that’s almost 9% greater than ÙCDM’s number. Riess and his colleagues, in turn, worked with Gaia and the HST and yet another method—a “cosmic distance ladder”, meaning they went step by step across the universe to calculate distances. Their Hubble Constant estimate was 74 km/s/Mps—as you see, in agreement with HoLiCOW.
Is this just measurement error? No, because this value for the Constant has popped up again and again as HoLiCOW and Riess’s colleagues have worked to refine their estimates. In fact, this is a statistically significant difference from the Planck number—a “five sigma” difference. That’s real enough that scientists have even given it a name, the Hubble “tension”. Some are beginning to speculate that there’s something else going on here, something fundamental that we haven’t come to grips with yet, something missing in the ÙCDM model of the universe that cosmologists have built over the years. That something acts to speed up the expansion of the universe.
At that Santa Barbara meeting, Riess put it this way: “If the late and early universe don’t agree, we have to be open to the possibility of new physics.” Yet the real measure of this tension may be that there are other numbers still. At that same Santa Barbara gathering, other astronomers were not 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 69.8km/s/Mpc. That’s well short of Riess’s 74, not much above the Planck 67.4 figure, certainly not a “five sigma” gap. A few months later, they had refined their estimate some more, to 69.6 km/s/Mpc.
So what’s going on? The short answer is really “we don’t know.” There’s clearly a gap that must be accounted for. But is it due to dark matter? Or to new physics? What?
There are rumours that at Santa Barbara, Madore sang these lines from an old song: “Clowns to the left of me, jokers to the right; here I am, stuck in the middle with you!“
To which I might add, “HoLiCOW!”
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun
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