Sorry, Astronomy Fans, The Hubble Constant Isn’t A Constant At All
“The fact that the Hubble expansion rate of the Universe changes over time teaches us that the expanding Universe isn’t a constant phenomenon. In fact, by measuring how that rate changes over time, we can learn what our Universe is made from: this was exactly how dark energy was first discovered.
But the “Hubble constant” itself is a misnomer. It has a value today that’s the same everywhere in the Universe, making it a constant in space, but it’s not a constant in time. In fact, so long as matter remains in our Universe, it will never become a constant, as increasing the volume will always make the density (and, a la Friedmann, the expansion rate) decrease. Perhaps it’s time to call it by its more accurate but rarely used name: the Hubble parameter. As it changes with time, it continues to reveal the very nature of our expanding Universe.”
Have you heard about the recent controversy over the Hubble constant? That some astronomers are arguing for a lower value while others are arguing for a higher value? Although that puzzle does present a legitimate conundrum, a very real issue is that this “constant” isn’t a constant at all, because it changes over time as the Universe expands!
I’d much prefer it to be called the Hubble parameter (and that its present value be called the Hubble parameter today), and that we relegate “Hubble constant” to where it belongs: the dustbin of history.
This Is Why We Aren’t Expanding, Even If The Universe Is
“As long as the Universe has the properties we measure it to have, this will remain the case forever. Dark energy may exist and cause the distant galaxies to accelerate away from us, but the effect of the expansion across a fixed distance will never increase. Only in the case of a cosmic “Big Rip” — which the evidence points away from, not towards — will this conclusion change.
The fabric of space itself may still be expanding everywhere, but it doesn’t have a measurable effect on every object. If some force binds you together strongly enough, the expanding Universe will have no effect on you. It’s only on the largest scales of all, where all the binding forces between objects are too weak to defeat the speedy Hubble rate, that expansion occurs at all. As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe."”
On the largest cosmic scales, everywhere we look, we see things moving away from us. The distant galaxies are receding not only from our perspective, but from one another. The Universe is expanding, a scientific fact that’s now nearly 100 years old. But we ourselves aren’t. Atoms remain the same size, as do our bodies, as do the scales of planets, solar systems, stars, and individual galaxies. Even groups and clusters of galaxies don’t appear to expand.
Why is that? Why aren’t we expanding, even as the Universe itself expands? Come get the physical explanation of the most profound phenomenon in the Universe.
Surprise! The Hubble Constant Changes Over Time
“If astronomers were more careful about their words, they would have called H the Hubble parameter, rather than the Hubble constant, since it changes over time. But for generations, the only distances we could measure were close enough that H appeared to be constant, and we’ve never updated this. Instead, we have to be careful to note that H is a function of time, and only today — where we call it H0 — is it a constant. In reality, the Hubble parameter changes over time, and it’s only a constant everywhere in space. Yet if we lived far enough in the future, we’d see that H stops changing entirely. As careful as we can be to make the distinction between what’s actually constant and what changes now, in the far future, dark energy ensures there will be no difference at all.”
The farther away you look in space, the more redshifted the light from the object you’re viewing appears. This implies that the Universe is expanding, and the rate of expansion, known as the Hubble constant, is something we’ve strived to measure for a very long time. While there’s a minor controversy over just what that expansion rate is today, whether it’s 67 or 73 km/s/Mpc, it’s perhaps surprising to learn that the expansion rate has changed over time. That means that the Hubble constant isn’t actually a constant at all! So why do we call it that? Because we’re evaluating how the Universe is expanding today. It’s constant everywhere in space, but has actually dropped. Interestingly enough, it will now asymptote to a finite, unchanging value. Many billions of years in the future, it will truly become a constant.
The fact that the energy density has changed over time means that the Hubble expansion rate has changed, too, and that the Hubble constant actually changes over time! Come find out how.
Why Cosmology’s Expanding Universe Controversy Is An Even Bigger Problem Than You Realize
“The question of how quickly the Universe is expanding is one that has troubled astronomers and astrophysicists since we first realized that cosmic expansion was a necessity. While it’s incredibly impressive that two completely independent methods yield answers that are close to within less than 10%, the fact that they don’t agree with each other is troubling.
If the distance ladder group is in error, and the expansion rate is truly on the low end and near 67 km/s/Mpc, the Universe could fall into line. But if the cosmic microwave background group is mistaken, and the expansion rate is closer to 73 km/s/Mpc, we just may have a crisis in modern cosmology.
The Universe cannot have the dark matter density and initial fluctuations that such a value would imply. Until this puzzle is resolved, we must be open to the possibility that a cosmic revolution may be on the horizon.”
Ever since we first learned that the Universe was expanding, scientists have worked hard to measure just how fast that expansion rate is. From that, combined with what makes up the Universe, we can learn how old the Universe is and what it was like in the past, as well as what it’s fate will be in the future. Yet the two groups that make independent measurements of that rate, from the cosmic microwave background and the cosmic distance ladder, have gotten inconsistent results. If the distance ladder team has made a mistake, everything will be fine with cosmology. But if that team is right and the microwave background team is wrong, there should be a crisis coming.
Why is that? Come find out why the biggest controversy in modern cosmology might be an even bigger problem than almost everyone realizes!
Scientists Still Don’t Know How Fast The Universe Is Expanding
“The uncertainties on these two methods are both pretty low, but are also mutually incompatible. If the Universe has less matter and more dark energy than we presently think, the numbers on the ‘leftover relic’ method could increase to line up with the higher values. If there are errors at any stage in our distance measurements, whether from parallax, calibrations, supernova evolution, or Cepheid distances, the ‘distance ladder’ method could be artificially high. There’s also the possibility, favored by many, that the true value lies somewhere in between.”
How quickly is the fabric of space expanding? That depends on how we ask the Universe. If we look at things like the leftover glow from the Big Bang or the large-scale clustering of galaxies, we get a consistent value of 67 km/s/Mpc. But if we look at individual galaxies through a variety of methods, we get a different consistent value: 74 km/s/Mpc. The uncertainties on each method are small and do not overlap, and a potential third way of measuring this (merging neutron stars) have problems and biases all their own. A generation ago, we argued whether the Hubble constant was 50 or 100; the answer turned out to be 70. Now, we argue over whether it’s 67 or 74… or, as a few people propose, that it’s again some value in between the two.
Scientists still don’t know how fast the Universe is expanding. Here’s what the controversy is all about.