The Hubble Tension and Early Dark Energy

In recent times I’ve posted quite a few times about the Hubble Tension and possible resolutions thereof. I also had polls to gauge the level of tension among my readers, like this one

and this one:

I’m not sure if these are still working, though, as I think I’ve reached the number of votes allowed on the basic free version of crowdsignal that comes with the free version of WordPress. I refuse to pay for the enhanced version. I’m nothing if not cheap. You can however still see the votes so far.

Anyway, there is a new(ish) paper on the arXiv by Mark Kamionkowski and Adam Riess that presents a nice readable introduction to this topic. I’m still not convinced that the Hubble Tension is anything more than an observational systematic, but I think this is a good discussion of what it might be if it is more than that.

Here is the abstract:

Over the past decade, the disparity between the value of the cosmic expansion rate directly determined from measurements of distance and redshift or instead from the standard ΛCDM cosmological model calibrated by measurements from the early Universe, has grown to a level of significance requiring a solution. Proposed systematic errors are not supported by the breadth of available data (and “unknown errors” untestable by lack of definition). Simple theoretical explanations for this “Hubble tension” that are consistent with the majority of the data have been surprisingly hard to come by, but in recent years, attention has focused increasingly on models that alter the early or pre-recombination physics of ΛCDM as the most feasible. Here, we describe the nature of this tension, emphasizing recent developments on the observational side. We then explain why early-Universe solutions are currently favored and the constraints that any such model must satisfy. We discuss one workable example, early dark energy, and describe how it can be tested with future measurements. Given an assortment of more extended recent reviews on specific aspects of the problem, the discussion is intended to be fairly general and understandable to a broad audience.

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Yet another Hubble Constant Update

The latest contribution to the ongoing debate about the Hubble constant is a new paper by Adam Riess and collaborators which you can find on the arXiv here. The abstract reads:

As you can see, this group is doubling down up on a high value for the Hubble constant. This longstanding discrepancy gives me an excuse to post my longstanding opinion polls on the topic.

First, would you go for a “high” (73-ish) or “low” (68-ish) value:

Second, do you think the discrepancy or tension is anything to get excited or even tense about?

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Cosmology Talks: Adam Riess on Cepheid Crowding and the Hubble Tension

Here’s another example from the series of cosmology talks being curated by Shaun Hotchkiss. In this one, esteemed astronomer and Nobel Prize winner Adam Riess talks about what he and collaborators considered to be the leading candidate for a systematic error in the SHOES measurement of the expansion rate of the Universe. This is “Cepheid crowding”, the possibility that background sources change our interpretation of Cepheid brightness, ruining one step in the SHOES distance ladder. Riess and collaborators devise a nice way to test whether the crowding is correctly accounted for and find that it is, so crowding cannot be the “explanation” of an error in the distance ladder measurement of H0. Riess also stresses that both the early and late universe measurements of H0 are now backed up by multiple different measurements. Accordingly, if the resolution isn’t fundamental physics, then no single systematic can entirely solve the tension.

P. S. The paper that accompanies this talk can be found on the arXiv here.

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A New Measurement of the Expansion Rate of the Universe – Adam Riess

Here’s a nice talk by Nobel Laureate Adam Riess delivered on May 11th at the Harvard-Smithsonian Centre Center for Astrophysics and is now available for you to watch at your leisure. It’s an hour long, but well worth watching if you’re interested in cosmology in general and in apparent tension between different determinations of the Hubble constant in particular.

Here’s the description of the talk, which is introduced first by Bach and Daniel Eisenstein:

The Hubble constant remains one of the most important parameters in the cosmological model, setting the size and age scales of the Universe. Present uncertainties in the cosmological model including the nature of dark energy, the properties of neutrinos and the scale of departures from flat geometry can be constrained by measurements of the Hubble constant made to higher precision than was possible with the first generations of Hubble Telescope instruments. A streamlined distance ladder constructed from infrared observations of Cepheids and type Ia supernovae with ruthless attention paid to systematics now provide 2.4% precision and offer the means to do much better. By steadily improving the precision and accuracy of the Hubble constant, we now see evidence for significant deviations from the standard model, referred to as LambdaCDM, and thus the exciting chance, if true, of discovering new fundamental physics such as exotic dark energy, a new relativistic particle, or a small curvature to name a few possibilities. I will review recent and expected progress.

And here’s the talk in full.

After watching the video you be interested in voting in my totally unscientific poll on the matter:

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Another Nobel Prize for Cosmology!

Just time in between teaching and meetings for a quick post on today’s announcement that the 2011 Nobel Prize for Physics has gone to Saul Perlmutter, Brian P. Schmidt and Adam G. Riess “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae.”

I’ve taken the liberty of copying the following text from the press release on the Nobel Foundation website

In 1998, cosmology was shaken at its foundations as two research teams presented their findings. Headed by Saul Perlmutter, one of the teams had set to work in 1988. Brian Schmidt headed another team, launched at the end of 1994, where Adam Riess was to play a crucial role.

The research teams raced to map the Universe by locating the most distant supernovae. More sophisticated telescopes on the ground and in space, as well as more powerful computers and new digital imaging sensors (CCD, Nobel Prize in Physics in 2009), opened the possibility in the 1990s to add more pieces to the cosmological puzzle.

The teams used a particular kind of supernova, called type Ia supernova. It is an explosion of an old compact star that is as heavy as the Sun but as small as the Earth. A single such supernova can emit as much light as a whole galaxy. All in all, the two research teams found over 50 distant supernovae whose light was weaker than expected – this was a sign that the expansion of the Universe was accelerating. The potential pitfalls had been numerous, and the scientists found reassurance in the fact that both groups had reached the same astonishing conclusion.

For almost a century, the Universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago. However, the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the Universe will end in ice.

The acceleration is thought to be driven by dark energy, but what that dark energy is remains an enigma – perhaps the greatest in physics today. What is known is that dark energy constitutes about three quarters of the Universe. Therefore the findings of the 2011 Nobel Laureates in Physics have helped to unveil a Universe that to a large extent is unknown to science. And everything is possible again.

I’m definitely among the skeptics when it comes to the standard interpretation of the supernova measurements, and more recent complementary data, in terms of dark energy. However this doesn’t diminish in any way my delight that these three scientists have been rewarded for their sterling observational efforts. The two groups involved in the Supernova Cosmology Project on the one hand, and the High Z Supernova Search, on the other, are both supreme examples of excellence in observational astronomy, taking on and overcoming what were previously thought to be insurmountable observational challenges. This award has been in the air for a few years now, and I’m delighted for all three scientists that their time has come at last. To my mind their discovery is all the more exciting because nobody really knows precisely what it is that they have discovered!

I know that Brian Schmidt is an occasional reader and commenter on this blog. I suspect he might be a little busy right now with the rest of the world’s media right to read this, let alone comment on here, but that won’t stop me congratulating him and the other winners on their achievement. I’m sure they’ll enjoy their visit to Stockholm!

Meanwhile the rest of us can bask in their reflected glory. There’s also been a huge amount of press interest in this announcement which has kept my phone ringing this morning. It’s only been five years since a Nobel Prize in physics went to cosmology, which says something for how exciting a field this is to work in!

UPDATE: There’s an interesting collection of quotes and reactions on the Guardian website, updated live.

UPDATE on the UPDATE: Yours truly gets a quote on the Nature News article about this!

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(Guest Post) Letter from America

Synchronicity can be a wonderful thing. Yesterday I mentioned the meeting of the Royal Astronomical Society that took place on January 10th 1930. The importance of this event was that it prompted Lemaître to write to Eddington pointing out that he had already (in 1927) worked out a solution of Einstein’s equations describing an expanding space-time; eventually this led to the widespread acceptance of the idea that Hubble‘s observational measurements of redshifts and distances of extragalactic nebulae were evidence that the Universe was expanding. 

Meanwhile, triggered by a recent article in Physics World, I have been having an entertaining electronic exchange with Bob Kirshner concerning a much more recent development about the expanding universe, namely that its expansion is accelerating. Since he’s one of the top experts on this, I thought “What better time  to have my first ever guest post?” and asked Bob if he would like to write something about that. He accepted the invitation, and here is his piece. 

 -0-

Twenty-first century astrophysicists (like Telescoper) are the wrong people to ask to cast your horoscope or maximize your feng-shui.  But even people who spend time in warm, well-lighted buildings staring at computer screens notice the changing seasons.  (This refers to conditions before the recent budget exercise.)  

For me, the pivot of the year comes right after the solstice, while the Christmas wrapping paper is still in the trash can.  Our house in Maine has a window facing south of east.  When the winter sun rises as far south as it ever does, a clear morning lets a blast of light come in one side, straight down the hallway and out the bathroom window. Househenge!  What does it mean? 

It means it is time for the American Astronomical Society’s big meeting.  This rotates its location from Washington DC, this year’s site, to other more-or-less tolerable climates.  Our tribe can mark the passage of the seasons and of the decades by this rhythm.  Never mind all that highfalutin’ stuff about the earth going around the Sun.  Remember that AAS in Austin? What year was that? 

In January of 1998, the cycle of the seasons and of available convention centers of suitable size put the AAS in Washington.  It was an exciting time for me, because we were hot on the trail of the accelerating universe.  We had some great new data from the Hubble Space Telescope (HST), a paper in the press, and Peter Garnavich, my postdoc, was going to give a talk and be part of a press briefing.  This was a big deal and we prepared carefully.  

Adam Riess, who had been my graduate student, was then a Miller Fellow at Berkeley doing the calibration and analysis on our data.  Adam’s notebooks were beginning to show troubling hints of cosmic acceleration.  I thought it would go away. Brian Schmidt, who had also been my student, was then in Australia,  calling the shots on this project.  He didn’t want to get out on a  limb over unpublished hints.  The idea of a cosmological constant was already making him sick to his stomach.  We agreed that in January of 1998, Peter got to say that the supernova data showed the universe was not decelerating very much and would expand forever.  That’s it.  Nothing about acceleration. 

Saul Perlmutter’s Supernova Cosmology Project also prepared a careful press release that reported a low density and predicted eternal cosmic expansion.  A report the next day in the New York Times was pretty tame, except for Ruth Daly speculating on the possibility of a low-density universe coming out of inflation models. Saul was quoted as saying, “I never underestimate the power of a theorist to come up with a new model.”  I have gathered up all the clippings I could find about who said what in Washington. (We used to call them “clippings”.) 

While a few reporters sniffed out the hints of cosmic acceleration in the raw data, in January 1998 nobody was claiming this was a solid result.  The paper from our team with the title Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant didn’t get submitted until March 13, 1998.  The comparable paper from the SCP was submitted September 8, 1998.  These are fine dates in the history of cosmology, but they are not in January.  It’s not for me to say when savants like the Telescoper were convinced we live in an accelerating universe, but I am pretty sure it wasn’t in January 1998.

In January 2009, the sun was once again shining right through our house.  It illuminated the American Physical Society newsletter kept in the upstairs bathroom. One of the features is This Month in Physics History.  If you want to find out about Bubble Chamber progress in January 1955, this is the place. Flipping through the January 2009 issue I was gobsmacked (American slang for “blown away”) to learn we were supposed to celebrate the anniversary of the discovery of cosmic acceleration.  Say what?  In January?  Because of the press releases that said the universe was not going to turn around? 

Being a dutiful type, a Fellow of the APS, and the oldest of the High-Z Team, I thought it was my job to help improve the accuracy of this journal. I wrote them a cheerful (on the third draft) letter explaining that this wasn’t precisely right, and, if they liked real publications as evidence for scientific progress, they might want to wait until March.  A volley of letters ensued, but not at internet speed.  The editor of APS News decided he had had enough education and closed the discussion in July.  The letters column moved on to less controversial matters concerning science and religion and nuclear reactors. 

The rising point of the sun came north, and then marched south again.    Just after the solstice, a beam of light flashed right though our   happy home. 2010!  Google alerts flashed the news.  More brouhahah about the discovery of cosmic acceleration.   Now in Physics World. I am depicted as a surly bull terrier in a crimson tenured chair, clinging desperately to self-aggrandizing notions that actual  publications in real journals are a way to see the order of events.  The philosopher, Robert P. Crease, who wrote this meditation, says he loves priority disputes.  He is making a serious point, that “Eureka!” is not exactly at one moment when you have an international collaboration, improving data sets, and the powerful tools of Bayesian inference at your command. 

But, even in the world of preprint servers, press releases, and blogs without restraint (I am talking about other blogs!), a higher standard of evidence is demanded for a real paper in a real journal.   A page in a notebook, an email, a group meeting, a comment after a colloquium or even an abstract in the AAS Bulletin (whipped up an hour before the deadline and months before the actual talk) is not quite what we mean by “having a result”.  I’m not saying that referees are always helpful, but they make the author anticipate a skeptical reader, so you really want to present a well-crafted  case.

If that’s not so, I would like to have my lifetime’s page charges refunded forthwith: that’s 250 papers x 10 pages/paper/ x $100/ApJ page = $250 000. Send the  check to my office.

So, Telescoper, how is your house aligned?  And why do the Brits put the drains on the outside when you live in such a cold climate?