More Unsettled Science

Now hear this:

We’re getting something wrong about the universe.

It might be something small: a measurement issue that makes certain stars looks closer or farther away than they are, something astrophysicists could fix with a few tweaks to how they measure distances across space. It might be something big: an error — or series of errors — in  cosmology, or our understanding of the universe’s origin and evolution. If that’s the case, our entire history of space and time may be messed up. But whatever the issue is, it’s making key observations of the universe disagree with each other: Measured one way, the universe appears to be expanding at a certain rate; measured another way, the universe appears to be expanding at a different rate. And, as a new paper shows, those discrepancies have gotten larger in recent years, even as the measurements have gotten more precise….

The two most famous measurements work very differently from one another. The first relies on the Cosmic Microwave Background (CMB): the microwave radiation leftover from the first moments after the Big Bang. Cosmologists have built theoretical models of the entire history of the universe on a CMB foundation — models they’re very confident in, and that would require an all-new physics to break. And taken together, Mack said, they produce a reasonably precise number for the Hubble constant, or H0, which governs how fast the universe is currently expanding.

The second measurement uses supernovas and flashing stars in nearby galaxies, known as Cepheids. By gauging how far those galaxies are from our own, and how fast they’re moving away from us, astronomers have gotten what they believe is a very precise measurement of the Hubble constant. And that method offers a different H0.

It’s possible that the CMB model is just wrong in some way, and that’s leading to some sort of systematic error in how physicists are understanding the universe….

It’s [also] possible … that the supernovas-Cepheid calculation is just wrong. Maybe physicists are measuring distances in our local universe wrong, and that’s leading to a miscalculation. It’s hard to imagine what that miscalculation would be, though…. Lots of astrophysicists have measured local distances from scratch and have come up with similar results. One possibility … is just that we live in a weird chunk of the universe where there are fewer galaxies and less gravity, so our neighborhood is expanding faster than the universe as a whole….

Coming measurements might clarify the contradiction — either explaining it away or heightening it, suggesting a new field of physics is necessary. The Large Synoptic Survey Telescope, scheduled to come online in 2020, should find hundreds of millions of supernovas, which should vastly improve the datasets astrophysicists are using to measure distances between galaxies. Eventually, … gravitational wave studies will get good enough to constrain the expansion of the universe as well, which should add another level of precision to cosmology. Down the road, … physicists might even develop instruments sensitive enough to watch objects expand away from one another in real time.

But for the moment cosmologists are still waiting and wondering why their measurements of the universe don’t make sense together.

Here’s a very rough analogy to the problem described above:

  • A car traveling at a steady speed on a highway passes two markers that are separated by a measured distance (expressed in miles). Dividing the distance between the markers by the time of travel between the markers (expressed in hours) gives the speed of the car in miles per hour.
  • The speed of the same car is estimated by a carefully calibrated radar gun, one that has been tested on many cars under conditions like those in which it is used on the car in question.
  • The two methods yield different results. They are so different that there is no overlap between the normal ranges of uncertainty for the two methods.

The problem is really much more complicated than that. In the everyday world of cars traveling on highways, relativistic effects are unimportant and can be ignored. In the universe where objects are moving away from each other at a vastly greater speed — a speed that seems to increase constantly — relativistic effects are crucial. By relativistic effects I mean the interdependence of distance, time, and speed — none of which is an absolute, and all of which depend on each other (maybe).

If the relativistic effects involved in measuring cosmological phenomena are well understood, they shouldn’t account for the disparate estimates of the Hubble constant (H0). This raises a possibility that isn’t mentioned in the article quoted above, namely, that the relativistic effects aren’t well understood or have been misestimated.

There are other possibilities; for example:

  • The basic cosmological assumption of a Big Bang and spatially uniform expansion is wrong.
  • The speed of light (and/or other supposed constants) isn’t invariant.
  • There is an “unknown unknown” that may never be identified, let alone quantified.

Whatever the case, this is a useful reminder that science is never settled.