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?

Gravitational waves discovery now officially dead

I tweeted this yesterday, but it deserves a blog entry: Gravitational waves discovery now officially dead : Nature News & Comment.

A team of astronomers that last year reported evidence for gravitational waves from the early Universe has now withdrawn the claim. A joint analysis of data recorded by the team’s BICEP2 telescope at the South Pole and by the European spacecraft Planck has revealed that the signal can be entirely attributed to dust in the Milky Way rather than having a more ancient, cosmic origin.

So, that’s that then.  As many will point out, this doesn’t mean the idea of observing primordial gravitational waves is dead, but it does remove what it would have implied (evidence for cosmic inflation, bubble universes, etc) back into speculative science, at least for now.  As someone who replied to my tweet said: science at work.

Most astrophysicists continue to believe that cosmic inflation is real, but of course, most astrophysicists before 1998 believed that the expansion of the universe was slowing, that it was only a matter of determining by how much it was slowing.  The discovery of dark energy caused a massive reset in those beliefs.  It awaits to be seen whether the beliefs about inflation will eventually require a similar reset.

Multiverse theories: “meta-cosmology”?

Level 2 multiverse
Level 2 multiverse (Photo credit: Wikipedia)

Marianne Freiberger reports on a discussion she had with Bernard Carr on whether or not multiverse theories are science.  He has a suggestion for how we should classify these theories.

With the possibility for indirect evidence in the future, maybe we shouldn’t dismiss the multiverse as mere speculation, especially since it has many features that are theoretically attractive. So attractive that some have even suggested we change the criteria of science in order to accommodate it. “The key question is: how crucial is testability?,” says Carr. “My view is that it is crucial; you do have to be able to test a theory to make it science.” He advocates classifying ideas like the multiverse in a special category he calls meta-cosmology: outside the present boundary of science, but not on the far end of fiction. “It’s a sort of intermediate state, a state of purgatory, before you’ve decided whether [something] is proper science or not.”

“Meta-cosmology” seems like an obvious dance around the term “metaphysics”, a term physicists seem to hate having applied to any theories they discuss.  But it seems like that label makes sense for speculations about unseen and untestable realms.  Of course, accepting it means accepting that physicists engage in philosophy, as least to some extent.  We should remember that many of today’s scientific concepts, such as atomism, began as metaphysical speculation.

Personally, if it makes cosmologists happier, I don’t see a problem with referring to multiverse theories as speculative science, provided that the “speculative” isn’t dropped.  Along the lines of Carr’s reference to calls some have made to change the criteria for established science, I think doing that would, at a minimum, do damage to cosmology’s credibility.

Cosmic inflation appears to have shifted from settled science back to speculation

You can get background on what I’m talking about in this post here and here.

Probably the best thing to do is let the experts weigh in on this.

It’s interesting to note that the empirical evidence from BICEP2 has never been called into question, only the interpretation of that evidence.  An interesting reminder, no doubt painful for the scientists personally involved, is that scientific evidence remains as much about interpretation of the evidence as the evidence itself.

The size of the observable universe is complicated.

Observable_universe_logarithmic_illustration (1)
Artist’s logarithmic scale conception of the observable universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic microwave radiation and Big Bang’s invisible plasma on the edge. By Pablo Carlos Budassi

The radius of the observable universe is often stated to be 46 billion light years.  From a certain point of view, this is true, but I think it’s a bit of a misleading statement.  Occasionally you also see people say that the observable universe is 13.8 billion light years in radius, which is also true, from a certain point of view.

How can both be true?  It has to do with two things: the finite speed of light, and the ongoing expansion of the universe.

Gazing upon the past

The speed of light is about 300,000 kilometers per second.  Whatever we see, we’re seeing in the past.  When you’re looking at an object across the room, what you’re seeing is on the order of nanoseconds in the past, but in the past nonetheless.  When looking at the sun, it’s about 8 minutes in the past, and when looking at the next closest star, it’s over 4 years in the past.

Of course, Proxima Centauri probably hasn’t changed much in the 4.24 years since its light started traveling to us, or even the Andromeda Galaxy since the light from it started traveling around 2.5 million years ago.  But when you start looking at objects billions of light years away, the evolution of those objects and the universe starts to become an important factor.

When we look out into the distant universe, we’re not seeing things as they are right now, but as they were in the distant past.  For cosmology, this is a good thing, because it allows astronomers to study the evolution of the universe, but it also means that we have limited insight into what the state of matter is billions of light years away right now.  Stars go through their lifecycles, galaxies form, merge, collide, and go through other transformations.

Right now?

Now, many relativity purists will say that the concept of “right now” applied to cosmologically distant objects is meaningless.  Strictly speaking, they’re right.  Of course, technically the concept of what you are doing “right now” as I type this post is also meaningless.

The concept of “right now” is useful to us because we’re close enough to interact, and we have a common objective measure of time to synchronize what we mean by “right now”, namely the time the Earth takes to rotate which we divide into numbered time slots, as well as the time the moon takes to orbit the Earth and for the Earth to orbit the Sun.  Of course the time the Earth takes to orbit the Sun is meaningless when observing regions and events that took place before there was a Sun and Earth.

Do we have any objective measure of time that is useful over cosmic distances?  Actually, we do.  The universe is constantly expanding, having started in an infinitessimally small and dense state.  As time passes, the average density of matter in space (averaged across hundreds of millions of light years) is decreasing.  Given the observed consistency of the early universe as seen in the cosmic microwave background, we can use the average density across cosmic distances as an objective measure of time since the Big Bang.

So, if you are a relativity purist, when I say “right now”, just substitute, “at the time when that distant region is at the same average density of matter that our region currently has.”

Our growing universe

As I mentioned above, the universe is expanding.  Actually, the better way to say that, is that space itself is growing.  Over billions of years, this becomes an important factor in considering distances.

Consider that the matter that generated the cosmic microwave background radiation that we’re currently seeing, when it generated that radiation, was only about 42 million light years away from the matter that eventually became us.  (To be clear, the cosmic microwave background was generated everywhere in the observable universe at the time, but the cosmic microwave radiation we actually detect today started traveling around 13.8 billion years ago.)

However, the matter that generated that cosmic microwave radiation is currently 46 billion light years away.  Why?  Because of the expansion of the universe.  That’s why many people will say that the radius of the observable universe is 46 billion light years.  But the light traveled for 13.8 billion years, across a distance of 13.8 billion light years while space was expanding around it, so that’s why others will say that the radius of the observable universe is 13.8 billion light years.

The furthest galaxies we can currently see are from light that has been traveling over 13 billion years.  When that light started traveling, those galaxies were only a couple of billion light years away from the matter that now makes up our galaxy.  Today, the matter that makes up those distant galaxies is over 30 billion light years away, and those distant galaxies have almost certainly radically evolved from what we’re now seeing.

So, how far into the universe can we see?  

The furthest thing we can currently see is the cosmic microwave background radiation generated from matter that is now 46 billion light years away, but we have limited insight into what that matters look like now.  We see galaxies that are composed of matter that is now over 30 billion light years away, but which have evolved in ways we can’t predict, except to say that on cosmic scales, they probably aren’t that different from those in our region.

There may be further complications in the future.  Astronomers may eventually be able to examine primordial gravitational waves from the time of cosmic inflation to reach conclusions about regions of the universe far more distant than the cosmic microwave background radiation.  If so, some people may conclude that the radius of the observable universe is much larger than any of the current numbers.

When we look out into the universe, we are looking into both space and time.  Given that, the size of the observable universe could be said to simply be around 13.8 billion years (not 13.8 billion light years, just 13.8 billion years).

You may disagree (if so, I’d love to read why in the comments), but regardless of what we consider the size of the observable universe to be, the important thing to keep in mind are the limitations of what we can actually observe in the observable universe.

“The Universe Should Not Have Lasted for More than a Second”: The limitations of scientific theories

Stan Hummel called my attention to, and asked for my thoughts on this article: Big Bang Theory Challenged –“The Universe Should Not Have Lasted for More than a Second”.

British cosmologists are puzzled: they predict that the universe should not have lasted for more than a second. This startling conclusion is the result of combining the latest observations of the sky with the recent discovery of the Higgs boson. according to Robert Hogan of King’s College London (KCL), who will present the new research today, 24 June at the Royal Astronomical Society‘s National Astronomy Meeting.

In the new research, scientists from KCL have investigated what the BICEP2 observations mean for the stability of the Universe. To do this, they combined the results with recent advances in particle physics. The detection of the Higgs boson by the Large Hadron Collider was announced in July 2012; since then, much has been learnt about its properties.

Measurements of the Higgs boson have allowed particle physicists to show that our universe sits in a valley of the ‘Higgs field’, which describes the way that other particles have mass. However, there is a different valley which is much deeper, but our universe is preventing from falling into it by a large energy barrier.

The problem is that the BICEP2 results predict that the universe would have received large ‘kicks’ during the cosmic inflation phase, pushing it into the other valley of the Higgs field within a fraction of a second. If that had happened, the universe would have quickly collapsed in a Big Crunch.

It’s tempting to come up with some humorous remark about this prediction, but this is important work.  When a scientific theory makes counter-factual predictions, it demonstrates that the theory is not yet complete.  Of course, I doubt there were many physicists who thought any of the models of cosmic inflation were necessarily complete, but demonstrating it is an important step.

But this also reminds me why I’m usually skeptical when people take these theories and extrapolate grand notions of reality from them (such as bubble universes).  Doing so gives us a possible version of reality, but any certitude about those predictions should be taken with a pound of salt.  There is just too much that is not known, and we often don’t know what we don’t know.

Indeed, every assumption a theory has to make that is not yet empirically testable, weakens the probability of that theory being truth.  And assuming that the structure posited by a successful theory continues unchanged beyond our observations is an assumption.  Often is it a valid one, but the further beyond observations you assume an unchanged structure, the more speculative it becomes.

If you read the history of science, you’ll see the trouble that such theories often run into.  We tend to celebrate the occasional successes of theoretical predictions, but overlook the legions of failure.  From what I’ve read, the theories that tend to be successful are the ones driven by empirical necessities, rather than possibilities.

None of this is to suggest that such theoretical work shouldn’t continue.  This kind of speculation is important.  The fact that most of it will ultimately prove fruitless has to be weighed against the fact that some of it will be fruitful, and there is no way to know ahead of time which category each competently crafted theory will fall into.  But we should be clear about the limitations of these theories, particularly their more speculative extrapolations.

The Big Bang’s Identity Crisis – The Nature of Reality

More bang for the buck? Credit: jeff_golden/Flickr, adapted under a Creative Commons license.

Think of the Big Bang, and you probably imagine a moment in time when matter, energy and space itself all burst into existence at once. Yet many astrophysicists now believe that the “Big Bang” was actually two distinct events: first the inaugural instant of space and time, and second the generation of most of the “stuff” that populates that space. So, which really deserves to be called the Big Bang?

via The Big Bang’s Identity Crisis – The Nature of Reality.

This article calls attention to a dispute I’ve noticed lately.  Some physicists insist that cosmic inflation happened before the Big Bang, others that the Big Bang was the moment of everything began.  Of course, if you accept the theory of eternal inflation, then there was no beginning, and the period of inflation ending is the point of the Big Bang.

Given how imprecise the term “Big Bang” actually is, how inappropriate of a term it is for describing what it applies to anyway, the debate strikes me a somewhat meaningless.  (No, as the article describes, I don’t have a catchier name than “Big Bang”.  That still doesn’t make it accurate.)

Personally, I’ve always thought of the Big Bang starting from the earliest moments of our universe that we can know anything about, and lasting until the cosmic microwave background was generated (after all, the CMB was often called the “afterglow” of the big bang), but I realize that isn’t how most cosmologists think about it.  Of course, an argument could be made that we’re still living within the Big Bang, since everything is still expanding and cooling, although dark energy complicates that assertion.

The debate does serve one purpose however.  It illuminates the stages of the early universe, which given how difficult these concepts are to describe, is actually a good thing.

Growing doubt that gravitational waves were actually detected

Nature has an article up describing the problems with the BICEP2 results that are now being identified by various scientists.  It’s actually the second one I’ve seen them publish on this.

The astronomers who this spring announced that they had evidence of primordial gravitational waves jumped the gun because they did not take into proper account a confounding effect of galactic dust, two new analyses suggest. Although further observations may yet find the signal to emerge from the noise, independent experts now say they no longer believe that the original data constituted significant evidence.

Researchers said in March that they had found a faint twisting pattern in the polarization of the cosmic microwave background (CMB), the Big Bang’s afterglow, using a South Pole-based radio telescope called BICEP2. This pattern, they said, was evidence for primordial gravitational waves, ripples in the fabric of space-time generated in the early Universe (see ‘Telescope captures view of gravitational waves‘). The announcement caused a sensation because it seemed to confirm the theory of cosmic inflation, which holds that the cosmos mushroomed in size during the first fraction of a second after the Big Bang.

However two independent analyses now suggest that those twisting patterns in the CMB polarization could just as easily be accounted for by dust in the Milky Way Galaxy12.

The BICEP2 team has reportedly been sticking to their guns, and many theoretical physicists initially downplayed the issues, so I’ve been reluctant to put too much credence to these stories.  But the doubts seem to be gaining more traction, and this caught my attention:

“I had thought that the result was very secure,” Alan Guth, the cosmologist who first proposed the concept of cosmic inflation in 1980, and who is at the Massachusetts Institute of Technology in Cambridge, told Nature after learning about Flauger’s talk. “Now the situation has changed.”

It’s still possible that when the BICEP2 team actually publishes their results, all of the issues will be addressed and results still found to be compelling.  But when the father of cosmic inflation loses confidence in the results, I’d say they’re on the ropes.  Inflation definitely may still be reality, but we might not have proof for it yet.

Americans may be more scientifically literate than evolution questions show

Dan Kahan has an interesting post showing that when Americans are asked the, “Did humans develop from earlier species?” question, it matters how it is asked.  As it’s usually asked, when people answer, they are often asserting a religious cultural identity.  But if it is asked with the qualifying “according to the theory of evolution”, the affirmative response rate goes up dramatically.

I take two things from this.  First, Americans are more scientifically literate than many people assume.  They just don’t always agree with that science.  Of course, many will insist that the lack of agreement is itself science illiteracy, but I think a person who understands the science, but still rejects it, is in a different category from someone who is clueless about that science.

My second takeaway though, is confirmation that the primary obstacle for people accepting evolution from natural selection and the big bang theory is their religious identification.

There’s a brief discussion in the post about how science teachers should address this.  I’m not sure it’s productive for them to try.  If someone learns about the science, but still rejects it, any attempt to “force” them to agree with evolution will probably only make them dig in deeper.  Sometimes, all you can do is lead a horse to water and hope it eventually drinks.

The theoretical preference for a timeless and eternal reality

Ethan Siegel has an excellent post up contemplating the various models of the timeline of the universe.

It’s only human to ask the most fundamental of all questions: where did all this come from? And we like to think we know the answer; it all came from the beginning.

But if you think about it for a little while, that simplistic answer — an answer that at first glance, might appear to be a tautology — presumes something very important about our Universe: that it had a beginning!

For a long time, scientifically, it didn’t appear that we knew whether that was true or not. The Universe could have had a beginning, before which nothing existed (or, at the very least, nothing as we understand it to be), or it could have existed eternally, like an infinite line extending in both directions, or it could have been cyclic like the circumference of a circle, repeating over and over again infinitely.

via How did the Universe begin? — Starts With A Bang! — Medium.

One thing I’ve noticed about this is that, at least in the last century or so, theoretical physics seems to have had a strong prejudice for the infinite universe.

It was Einstein’s preferred model, so much so that he introduced the cosmological constant to avoid dealing with the otherwise mathematical need to explain either an expanding or contracting universe.  He later referred to that move as his greatest blunder.  (Even though it ultimately proved prescient with dark energy, but for the wrong reasons.)

Once it became apparent that we were in an expanding universe, Fred Hoyle introduced the steady state theory to explain away any possibility of a beginning.  His theory remained a respectful alternative to the big bang until the discovery of the cosmic microwave background, a prediction unique to the big bang.  But Hoyle never accepted defeat, holding on to steady state until the end.

Shortly after a need for cosmic inflation was realized, eternal inflation became a popular assumption.  Even if eternal inflation ultimately ends up not being accurate, and our universe appears to have a beginning, there will be the various multiverses theories to make the whole works timeless and eternal again.

Whenever theoretical physics has little or no observational information, it seems to default to reality being timeless and eternal.  To a large extent, I can understand this, since timeless infinity is mathematically simpler than a finite or circular timeline.

But since observational data is generally assessed within the framework of theoretical assumptions, it seems like this should make the physics community a bit uneasy, a bit on guard that this preference could blind them for a while to contrary empirical evidence or even mathematical implications.   I see posts like this one from Siegel as a good sign that there are physicists resisting this temptation, but I it seems like I read a lot more from others assuming infinity.