One of the biggest surprises that Einstein had when exploring his new General Theory of Relativity was discovering that it predicted a dynamic universe. Einstein had conceived of the universe as being static—galaxies moving around relative to each other, like molecules in a gas, but conserving the same density. The prediction of his theory, however, required that the universe be either expanding or collapsing. Assuming that this was some kind of error, Einstein introduced what he called the cosmological constant to enforce stasis on his calculated universe. Not long after, astronomer Edwin Hubble reported evidence that the universe was in fact expanding: light from galaxies tended to be red-shifted, implying that they were moving away, and the more distant the galaxy, the greater the red-shift, i. e. the faster it was moving away. In reaction to this, Einstein called his invocation of the cosmological constant his “biggest blunder” (though current cosmologists disagree on that point).
A challenge for astronomers and cosmologists is to try to accurately measure the Hubble constant, that is the rate of recession of a galaxy as a function of its distance from us. A lot about the history and the fate of the universe is riding on this value. From the time of its discovery, this ratio of speed of recession to distance has appeared to be a constant. One of the more important purposes of the Hubble Space Telescope was to provide the necessary data to provide a value of the Hubble constant to an unprecedented precision, and it did that. However, another team using data from the cosmic microwave background (CMB) and other assumptions have produced another very precise measurement of the Hubble Constant that disagrees with the other measurement by about 9 %, a big discrepancy well outside the margins for error of either measurement. In the field, this disagreement has come to be known as “the tension.”
We’ll look at how these measurements were performed and where the discrepancy might arise, as well as the comments (of course!), over the fold. But first, here’s a word from our sponsor:
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So how were the measurements performed? The first was based on a large number of measurements of phenomena that serve as “standard candles,” that is, depending on the phenomenon, you know what the absolute brightness of the phenomenon is. Comparing the observed brightness with the absolute brightness gives the distance. Then you measure the red-shift, and that gives the speed at which it’s traveling. The first of these phenomena are a variety of variable stars called Cepheids. The brightness of such stars oscillates at a constant frequency, and the absolute brightness increases with the frequency in a predictable way. However, as distances increase, it becomes progressively harder to see the Cepheids, which means it’s necessary to resort to something else. That something else is type Ia supernovae, which, like Cepheids, have a predictable absolute brightness. After collection many such observations from the ground, from the Hubble Space Telescope, and from the European Space Agency’s (ESA) Gaia spacecraft, they have determined a value of 73.5 kilometers per second per megaparsec (with 2.4 % uncertainty). (A megaparsec is about 3.3 million light years.)
The other measurement involves the map of the CMB radiation (the echo of the big bang) obtained by another ESA satellite, Planck. The map (at right) shows variations in frequency with high resolution. Frozen into this map are variations in the density of the early universe (380,000 years old).
It is possible to model the early universe by creating the expected pudding of charged particles, and determining the characteristics of sound waves moving through the pudding using cosmology’s standard model until the density reflected in the observed CMB map is recovered. From such a calculation, one gets a highly constrained value of the Hubble constant, of 67.3 kilometers per second per megaparsec (with 1 % uncertainty).
These two measurements can’t both be right, but each camp is vaunting the rigor of its methods while throwing shade on the other camp. Camp 1 asserts the consistency of its measurements while reminding everybody about the jury-rigged nature of the cosmological standard model used by Camp 2. Camp 2, on the other hand, points out that it would be easy to make errors in calibrating those standard candles if Camp 1 didn’t properly account for the light from background stars.
One potential explanation could lie in Einstein’s “blunder,” the cosmological constant. In the 1990s, observations on distant supernovae were used to discover that the rate of expansion of the universe was actually speeding up. This is contrary to what was expected: gravity applies an attraction between all the matter in the universe, and so you’d think gravity would put the brakes on matter flying apart. It has been presumed that the acceleration of expansion is due to the presence of "dark energy" in the universe, and that this dark energy can be accounted for by adjusting the cosmological constant to the correct value. Recognizing that the Hubble constant value calculated from the CMB observations, and that those values reflect the early universe, when expansion would have been slower, you might think that the smaller value obtained from these observations sort of fits. the CMB value is smaller because the acceleration hadn't started yet, and the other value comes from more local observations, with the current expansion rate. Unfortunately, this explanation doesn't work because the acceleration rate isn't large enough to account for the 9 % difference.
So it's back to the drawing board. New satellites will be launched in the next decade that will be able to obtain even more precise measurements on both red-shift data as well as obtaining estimates on the content of dark energy. Meanwhile, theorists can attack the discrepancy to determine if (1) there's a problem with the cosmological standard model or (2) there is some effect from dark energy that hasn't been accounted for. Unfortunately, it's not likely this discrepancy will be resolved soon, but it will probably be resolved before the end of the universe.
And now, on to the comments!
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