Is cosmic inflation settled science?

Ethan Siegel at Starts With a Bang has a post up arguing that the multiverse must exist.  His reasoning has to do with cosmic inflation.  Inflation is the theory that the universe expanded at an exponential rate in the first billionth of a trillionth of a trillionth of a second of the big bang timeline.  After that, space started expanding at a slower rate along the lines we see today.

Inflation was originally developed as a theory to explain why the universe seemed flat (in terms of spacetime), why temperature fluctuations throughout the early universe were so consistent, and a few other things such as the absence of magnetic monopoles.  These motivations for the theory seem important.  I think they should be remembered when evidence for the theory is being discussed.  In other words, can we cite the motivations for the theory as evidence for it?

Anyway, Siegel notes that, due to quantum uncertainty, the particle physics that led to inflation ending would not have been a strictly deterministic event.  Therefore inflation may not have ended everywhere in space at the same time.  Our universe may be a bubble of non-inflation in a sea of inflating space.   And if there is one bubble, there are likely others, other universes, the multiverse.

But all of this seems to hinge on the idea of cosmic inflation, specifically a variant of it called eternal inflation.  And here’s my issue.  The majority of physicists do seem to accept that the theory of inflation is true, but not all of them.  And the evidence for it seems to be a combination of the original problems the theory was developed to solve and circumstantial evidence.   Some notable physicists, including Paul Steinhardt, one of the theory’s early supporters, seem to think it actually creates more questions than it answers.

So, should we consider inflation settled science?  This seems like an example of an unsettling trend in physics in recent years of accepting theories that can’t be empirically tested.  Given that a huge swath of similar theories in particle physics were reportedly invalidated by the LHC results (or rather, non-results), it seems like a very questionable strategy, an abandoning of a key aspect of scientific investigation that been successful for centuries.

Science has credibility for a reason.  That reason is the value it puts on testing every proposition.  Talking about untestable theories as though they’ve been validated seems to put that credibility in jeopardy.  There’s a danger that the public will see start to see theoretical physics as metaphysical navel gazing.

There is also a danger, identified by Jim Baggott some years ago, that many scientists may simply not look at alternative theories because they think inflation has solved the issue, or that they may eschew some speculative theories just because they’re not compatible with inflation.  But if inflation is still really just a speculative theory, then they’re giving up on one speculative theory because it’s not compatible with another speculative theory, perhaps cutting off a fruitful line of inquiry.

It may turn out that inflation does eventually pass some test we simply haven’t thought of yet.  Or we may eventually figure out a way to test the idea of bubble universes.  But until we do, talking as though these are settled issues makes my skeptic meter jump through the roof.

Physics has a reputation for being a very hard science.  Sometimes I wonder how warranted that reputation really is, at least in the theoretical branches.

Unless of course I’m missing something?

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.

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.