Is cosmology in crisis?

In past posts, when I’ve written about the expansion of the universe, I’ve generally referred to the rate of that expansion, the Hubble constant, as being around 70 km/s/megaparsec, that is, for every megaparsec a galaxy is distant from us, it’s moving away at 70 kilometers per second faster.  So a galaxy 100 megapasecs away is moving away at 7000 km/s, and one 200 megaparsecs away at 14000 km/s.

But I have to admit this was an oversimplification.  70 km/s was actually a rough and rounded averaging of two measurements for the expansion, one taken using the cosmic distance ladder, and the other using observations of the cosmic background radiation.  The former currently yields about 74 km/s, and the latter about 67 km/s.

Everyone for a long time thought this difference was just a measuring artefact that would eventually be smoothed out.  Everyone is turning out to be wrong.  As this news story discusses, the two measurements have been refined extensively.  Confidence in these individual measurements are pretty high, and the margins of error don’t overlap.

In other words, either one or both of these methods has assumptions in it that are wrong, or there is something completely unexpected going on in the universe that cosmologists haven’t yet accounted for.  For most scientists, this is a reason for excitement.  This kind of issue typically leads to new insights.

However, it’s led to a debate that someone has been asking me to comment on.  Bjorn Ekeberg, a philosopher of science, has focused in on this problem, along with others, to assert that cosmology has some big problems, calling into question the overall big bang cosmology.  This drew a response from cosmologist and science writer Ethan Siegel pushing back against Ekeberg’s claim and accusing him of being anti-science.  Ekeberg has responded accusing Siegel of being a “temple guard” for big bang cosmology.

Name calling aside, who’s right here?  Not being a professional physicist, or knowledgeable enough to read raw physics papers, my comments are inevitably based on what various science writers have provided.

But in considering Ekeberg’s position, it’s worth reviewing the evidence for the overall big bang model.  Physicists in the 1920s figured out that, under general relativity, the universe could not be static.  It had to be either expanding or collapsing.  If it was expanding, it was smaller yesterday than today, and smaller the day before.  Following that back led to a period in the past where everything was all bunched up together, very dense and very hot.  The physics of the early universe could be mathematically deduced and predictions made.  (This led Einstein to fudge his equations a bit by adding a cosmological constant, making the universe it predicted static.)

Then in the late 1920s, Edwin Hubble discovered that the light from every galaxy beyond a certain distance was red shifted, with the amount of red shift being proportional to the distance.  Red shift is a doppler effect that happens when something is moving away from the observer.  Hubble had discovered that the universe is indeed expanding.  (Einstein concluded that the cosmological constant was his biggest blunder.)

Still, cosmology was slow to just accept the big bang model.  (It didn’t help that Hubble’s early estimates of the age of the universe had it younger than geologist estimates of the Earth’s age.)  It continued to be debated for decades, until the discovery of the cosmic background radiation in the 1960s, which provided evidence for the calculations of the physics of the early universe.  That was enough for most cosmologists.  The big bang became settled science.

As a lay person, reading this through the translations of the experts, classic big bang cosmology seems pretty solid.  I think Ekeberg, by implying it isn’t, oversells his thesis.  But it’s worth noting that this settled version doesn’t get into what caused the big bang in the first place.

Ekeberg also has issues with the ideas of dark matter and dark energy.  My understanding of these terms is that they’re essentially place holders, labels for our ignorance.  So criticism of them as theories has always struck me as premature.

The most often touted alternative to dark matter is MOND (modified Newtonian dynamics), but no simple modification to the equations seem able to account for all the observations.  Whatever is causing the rapid rotation of galaxies and other intergalactic effects seems to require something that is present in varying densities or intensities.  Dark matter may eventually be so different from matter as we understand it that the word “matter” might not be appropriate, but until then, the term really just refers to something mysterious causing varying gravitational effects.

This seems even more true for dark energy.  The fact that, against all expectations, the expansion of the universe is actually accelerating rather than decelerating, has to be caused by something, some form of unknown energy.  (Ironically, dark energy has resurrected Einstein’s cosmological constant.)

Granted, it does seem unnerving that this results in 95% of the matter and energy in the universe being unobservable and unaccounted for.  It’s easy to take this number and other measurement issues and accuse cosmologists of not knowing what they’re doing.  Easy, but I think facile.  The widely accepted theories that we now have are grounded in observation.  Anyone is free to propose alternatives, but to be taken seriously, those alternative have to account for at least as much of the data as the current theories.

I do think one area where Siegel is overconfident is cosmic inflation.  I’ve written about the concerns on this before.  Some version of inflation might turn out to be true, but I think his stance that it’s a settled issue isn’t justified yet.  And the fact that a significant portion of physicists are starting to question inflation, including some of its earliest supporters who now say it generates more issues than it solves, should make the rest of us cautious in our stance toward it.

So, does cosmology have issues?  Of course, and Siegel admits as much.  But is the overall big bang cosmology model in crisis as Ekeberg seems to contend?  I think this is vastly overstating the issues.  But only time and the data will tell.  Of course, this controversy will likely lead to more sales for Ekeberg’s book.

What do you think?  Is the overall big bang model in trouble?  Or is this just about fine tuning the details, such as the age of the universe?  If it is in trouble, what might replace it?

Dark matter might cause neutron stars to collapse into black holes

English: Vector compound of File:Neutron_star_...
(Photo credit: Wikipedia)

ratamacue0 called my attention to this interesting article on the possibility of dark matter “eating” neutron stars: Dark matter: Devourer of stars | Ars Technica.

Neutron stars are collapsed stars that have used up all of their fusion fuel.  Typically what happens at that point in a star’s life is that they collapse, but the extent of the collapse is largely a factor of how much mass they had.  A star the size of our sun will collapse into a white dwarf (dense but still composed of atoms with electron clouds), but a heavier star will often collapse into a much denser neutron star (the gravity has crushed the electron clouds out of existence with only neutrons left, at least at their core).  Heavier stars yet will collapse into black holes (where the gravity overwhelms all repulsive forces between particles and causes the whole structure to collapse into an infinitely dense point, a singularity).

Apparently, the problem is that there aren’t as many neutron stars at the center of the galaxy as there should be according to astrophysical predictions.  One possible explanation is that, over time, heavy neutron stars attract too much dark matter into their cores, and that the additional mass collapses them into black holes.  It’s an interesting theory, but as the article describes, it’s just one of many possibilities.

But reading this article made me wonder how much of the mass of the super-massive black hole at the center our galaxy might be composed of dark matter.  Or if it’s possible for dark matter in other regions to collapse into a black hole without ever going through the star stage.  It seems like it would depend on to what extent dark matter interacts with itself.

And that gets the the problem with any theory involving dark matter.  We just don’t know what it is yet.  Dark matter is only detected by its gravitational effects.  No one has yet managed to detect it in any other way.  There are lots of ongoing experiments to do just that.  Hopefully one of them will eventually make that detection, and the nature of it will tell us more about what appears to make up the majority of the matter in the universe.

The movie ‘Interstellar’ and wormholes

The other day, I did a post on interstellar exploration which linked to one by Sten Odenwald on the problems with interstellar travel.  Well, he posted some follow-up remarks, expressing some surprise at the response, doubling down on the aspects of the limitations of interstellar travel he identified, and urging people to be optimistically realistic.  (I predict he’ll get a similar response to this post.)

One thing I wanted to add to the remarks I made in my post, is that I don’t oppose research into possible faster than light solutions.  I just think we have to realistic about their prospects.  This subject is coming up again with the release of the movie ‘Interstellar’ tomorrow.

It sounds like a big part of the movie’s plot is going to involve wormholes.  These are actually theoretical concepts, and the movie had a heavy weight physicist, Kip Thorne, consulting to make sure they got it right.  (Thorne is actually releasing a book about the physics of the movie.)

As I said in my earlier post, these faster than light concepts are extremely speculative.  To understand how speculative, you might be interested in this write up by Paul Halpern at the Starts With A Bang blog.  The TL;DR is that traversable wormholes require something called “exotic matter” to produce “negative energy” to keep them from instantly collapsing.  Exotic matter has not yet been observed in nature.  Of course, that doesn’t mean it doesn’t exist, or that it couldn’t conceivably be manufactured.

But then there’s this:

Even if exotic matter is identified and put to use, there is another obstacle to traversable wormhole construction — the enormous amount of ordinary matter required. Researchers estimate that one would need a glob of mass comparable to millions of suns. Clearly, wormhole construction is not in the cards for the foreseeable future.

It’s always possible someone will find a way around these difficulties.  We don’t know what science will discover in centuries to come.

But if we’re doing scientific speculation, the probabilities are that exploring the stars will happen on far longer time frames than we’re used to now, and our best bet may be engineering ourselves to cope with those time frames.  Far out?  Sure.  But anywhere near as far out as harnessing the mass of a million suns to create a wormhole?

Ask Ethan #12: How far is the distant Universe? – Starts With A Bang

Ethan Siegel’s blog, Starts With a Bang, is one of the best science astrophysics blogs on the web.

Ask Ethan #12: How far is the distant Universe? – Starts With A Bang.

In this post, he tackles an issue I often see a lot of confusion over, even from science reporters.  When talking about extreme distances, to the edge of the observable universe…well, it’s complicated.  If you’re not familiar with terms like comoving distance or light travel time, or don’t understand how someone can authoritatively say that a galaxy is 33 billion light years when the age of the universe is only 13.8 billion years, then you might want to check this post out.