The Hubble Constant Isnt

The Big Bang description of our universe is heavily based on mathematical models that do not seem to stand up well to observations. Is the Big Bang Model doomed?

Published on 3rd May, 2019

Hey everybody and welcome back to another Deep Astronomy vlog post, I’m Tony Darnell from Deep Astronomy dot space and I dunno about you, but lately I’ve been seeing all kinds of headlines streaming through my newsfeed about the universe expanding faster than astronomers thought. Have you seen these: Mystery of the Universe’s Expansion Rate Widens With New Hubble Data, New Hubble Measurements Confirm Universe Is Outpacing All Expectations of its Expansion Rate, The Universe Seems to be Expanding Beyond all expectations, Hubble shows the universe is expanding faster than we thought. Stuff like that? I love how the news cycles around astronomy stuff works: someone publishes a paper on, sometimes refereed, sometimes not, then an intrepid reporter scanning the astro-ph section sees a catchy title and writes an article about it to try and scoop everybody else. It also works slightly differently sometimes, In the case of this story, the way it works is an astronomer that works for a university or institution gets his paper published in a refereed journal and the organization he or she works for issues a press release and then an explosion of articles appears basically everywhere. The thing is, this recent headline about the faster than thought expanding universe isn’t really all that new. For years there has been a discrepancy between two very key ways of measuring the universal expansion rate -- also called the Hubble Constant, but should really be called the Hubble Parameter because it is not a constant, it changes with time - and this discrepancy really has astronomers in a tizzy. If you’ve been watching Space Fan News for any length of time then you know we talked about this in SFN 260 back in March where I outlined the issue in more detail so go check that video out to learn more because I laid out the problem clearly there and I don’t want to rehash that now in this video. But what’s different here - and its the only thing that’s different from what I talked about in SFB 260 is Dr. Adam Reiss from STScI used the Hubble space telescope to measure 70 Cepheid variable stars in the Large Magellanic Cloud to try and make sure that the way we are measuring these important yardsticks is being done properly. For those who don’t know, Cepheid variables change brightness is just such a way that when they change brightness, we can tell how intrinsically bright they are. And when we know how bright they are when they are up close we can figure out how far away they are when we see them in our galaxy and nearby galaxies which are about 100,000 light years away. Then, and this is important, these variables that Adam Reiss measured can be compared with farther away ones in galaxies that also have redshifts to. We need both the brightness of something AND it’s velocity to get the Hubble Parameter number. km/sec is a speed and megaparsec is a distance so there you go... So before this paper, we knew how intrinsically bright Cepheid variable were to within 2.2%. Now, after this paper, we know the to within 1.8%. Good, better, best I guess but at least we haven’t totally screwed up the cepheid variable measurement. This refinement meant that the Hubble Parameter changed a little bit which is where all those headlines were blabbing about and the universe is a little bigger because its expanding faster, all kinds of things were flying around the internets. This also means that the number we get for the expansion rate of the universe using the Type 1a supernovae and Cepheid variable technique is known a little better. That number is 74 km/sec/megaparsec. And it’s different that what the early universe guys get when measuring the CMB, at 67 km/sec/megaparsec, a difference of about 9%. And they say they know their number to well within that. Clearly, someone is wrong, the answer to the universe expansion can’t be 67 kilometers per second per megaparsec and 74 kilometers per second per megaparsec. So somebody is wrong. That’s a nine percent difference in a world where everyone is with 2% sureity of their answer. So it’s kind of a big deal. The way they like to put it is that this discrepancy has about a one in a hundred thousandths of a chance of being a fluke. It’s real and they need to figure it out. I think what this points out the most though is just how non-constant the Hubble constant is. These observations are hinting that maybe the universe hasn’t expanded at the same rate all the time. In fact, if dark energy is the culprit for the accelerating universe, then maybe some parts of the universe are accelerating faster than others. Wouldn’t that be weird? There’s nothing about what we know in cosmology that says we couldn’t have a universe that expanded slower in the beginning, speeded up again later, then slowed down some more, and on and on. There’s also the possibility that different regions are expanding at different rates (although I don’t think we’ve seen evidence of that) but it is possible. As I said in SFN 260, my money is on LIGO to help us shed more light on this problem by looking at neutron star collisions, but I also think that it’s OK that both camps of astronomers got different values for the expansion rate of the universe. In fact, given what we’ve learned about the recent universe, I’d be more surprised if the universe never changed expansion rates. But let me leave you with this thought: Cosmology - the branch of science that tries to understand and describe the universe as a whole, relies quite heavily on mathematical models to explain things. There aren’t many observations available, in fact, there’s really only two: there’s the observation that the universe is expanding and accelerating; and there’s the discovery of the Cosmic Microwave Radiation that is described as the remnant of the Big Bang. Everything else used in cosmology relies extremely heavily on assumptions and tweaks. Just these two observations - the only two we have of the universe as a whole - required some serious tweaking to the Big Bang model. It’s starting to look more and more like the mathematical description of our universe doesn’t stand up to observation very well. Why do I say that? Observations that galaxies rotate strange throughout the universe required us to make up something called Dark Matter that constitutes 25% of the matter everywhere, and the acceleration of the universe requires us to believe that there is an energy that permeates 70% of the universe. Neither of these ideas are based on anything other than trying to reconcile observation with the Big Bang model. With every new observation we make, we get new tweaks to the cosmological model of the universe that in the end leaves us with the notion that we must accept that 95% of our cosmos is unobserved and possibly unknowable. And more crucially, the Big Bang model doesn’t have anything to say about the Big Bang itself. If that’s not a model in crisis, I’m not sure what is.