CERN Accelerating science

An interview with Nobel Laureate Adam Riess

by Panos Charitos

An interview with Nobel Laureate Adam Riess (Professor of Astronomy and Physics at the Johns Hopkins University) who was jointly awarded the 2011 Nobel Prize for Physics, with Saul Perlmutter and Brian P. Schmidt, for discovering the accelerated expansion of the Universe. The acceleration of the universe is a startling result that completely changed modern physics revealing that the majority of the universe’s mass-energy was of a completely unknown nature. Nima Arkani-Hamed was right to characterize the discovery of the Higgs and the discovery of the accelerated expansion of the Universe as the two most dramatic moments in the physics of the 21st century. 

We discuss with Riess about his previous work on using Type Ia supernovae to measure the expansion rate of the universe, the steps that lead to this discovery and the role that particle physics could play in interpreting this result. Finally, we ask him about the Hubble constant tension – a topic where he is actively involved – and whether this signals a new era for modern cosmology.

1) What was the path that led you to choose astronomy as a career – were you interested in science from a young age?

As a kid I was fascinated by the big questions about what is out there and for how long our universe existed. These are questions that one could either ask in a religious context or a scientific context and the latter appealed to me.

As a graduate student I was fascinated to learn that the universe is expanding and that we could quantitatively measure this expansion and determine its age. So after my junior year in high school, I spent a month at New Jersey Governor’s school of science where I took my first course on Einstein’s theory of special relativity. When Jim Supplee started explaining the concepts of space contraction and time dilation I knew that this is what I wanted to follow and I continued in MIT’s physics class in 1988 and then to Harvard for my PhD with Bob Kirshner and William Press.  

2) Could you tell us a few words about "the discovery of the accelerating expansion of the universe through observations of distant supernovae." for which you won the Nobel Prize in Physics in 2011? 

Today we are able to make very precise measurements of the expansion rate of the universe by measuring the distances and redshifts of supernova explosions of Type Ia. The shift in a supernova’s spectrum due to the expansion of space gives its redshift (z) and the relation between redshift and distance is used to determine the expansion rate of the universe. Supernovae with greater redshifts, lying at greater distances, reveal the past expansion rate as their light was emitted at an epoch when the universe was younger. 

Supernovae Type Ia were the suitable candidate for these measurements as you need objects that are very luminous (thus can be observed even when they are very far) and highly uniform (so that intrinsic scatter doesn't blur the signal). Supernovae Type Ia are the most luminous of the common supernova types, peaking at 4 billion solar luminosities, and thus allowing us to look at extreme large distances. However things can get confused as some are intrinsically more luminous than others requiring careful analysis to precisely estimate their distances. Moreover, variations in the amount of dust along the line of sight further complicates these measurements. Part of the success that led to this discovery, comes from the development of instruments and methods to tackle these issues and to increase the precision of these measurements, particularly by developing new algorithms and also from the use of large format CCDs.

Measuring with accuracy the distance and redshift of Supernovae of Type Ia was the goal of a campaign that we launched 1994. Furthermore, by measuring these quantities for objects that lie even further away we can infer how fast the universe expands at a certain time in the past, perhaps even billions years ago. By comparing the expansion rates at two different epochs of the universe we can estimate the expansion rate of the universe and how it changes over time.

3) What was the first result? 

We made this comparison in 1998, using a sample of 15 SNIa, and to our surprise we found that instead of decreasing the expansion rate was speeding up. In other words, the expansion of the universe is accelerating. The result was enabled through our use of a set of 34 nearby SNe (17 supernovae from the Calan/Tololo Survey that both teams used, another 17 from my thesis and my Snapshot paper). 

      

4) What was the original motivation for these measurements? 

We wanted to measure the expected deceleration of the universe at larger scales. The hope was to find evidence for some kind of extra matter that theorists predicted might be out there. Back in the 1990s the assumption was that we live in a dense universe, governed by baryonic and dark matter but astronomers could only observe 30% of the expected matter. Therefore we looked at larger scales to measure the deceleration rate due to gravity as this could give us a hint about the universe’s total mass. But instead of decelerating we found that the universe was expanding at an accelerating rate. 

Knowing how the expansion decelerated we can predict the amount of mass the universe must have. The higher the mass of the universe the more gravity should pull against its expansion leading to a deceleration of the expansion rate. But what we measured was stunning! The only way to match the measured change in the expansion rate was to allow for some type of “negative” mass. Since there is no such thing as negative mass the result could be interpreted if the universe instead of decelerating is speeding up its expansion. 

5) What was the first reactions from your colleagues when the result was announced?

 

That our result was wrong (laughs). There were understandably different reactions but the fact that two independent teams [the Supernova Cosmology Project and the High-Z Supernova Search Team] were measuring an accelerating expansion rate and the independent confirmation from measurements of the Cosmic Microwave Background made it clear that the universe is accelerating. 

 

We reviewed all possible sources of errors including the presence of some unknown astrophysical process but these were ultimately ruled out. I should add that our previous work in analysing and removing the effects of interstellar dust was also helpful in this respect. Barring a series of unrelated mistakes, we were looking at a new feature of the universe.

 

There were other puzzles at that time in cosmology that the idea of an accelerating universe could also solve. The so-called “age crisis”, as many stars were looking older than the age of the universe, was one of them. This meant that either the stellar ages are too high or that there is something wrong with the age of the universe and its expansion. This discrepancy could be resolved taking into account an accelerated expansion and a new value for the Hubble constant. So everything fit together.

 

6) How this seemingly odd result can be interpreted? 

 

The result reminded us of the cosmological constant that Einstein famously introduced in 1917 to get a static universe though it is clear that we are dealing with something new. 

 

One idea is that the cosmological constant can be linked to the vacuum energy but we know that vacuum energy can’t be the final answer. If one sums the contributions from the presumed quantum states in the universe it gets to be an enormous number for the expansion rate; about 120 times higher than what is actually occurring. This acceleration rate is so high that it would have ripped apart galaxies, stars, and planets, before anything formed. So the fact that we observe structures in the universe - and that we exist - tells us that that calculation is grossly inaccurate.

 

Today we are trying to measure more precisely this expansion rate. This accelerating expansion can be due to what we broadly refer to as dark energy, that is strong enough to push the entire universe but its source and its physics remain unknown. It is an ongoing area of research.

 

7) By which other methods we try to measure the rate of this expansion? 

 

Today there is a vast range of approaches, using both space and ground experiments, for measuring the acceleration rate of the universe. A lot of work is ongoing to identify more SN Type Ia and measure their distances and redshifts with higher precision. 

 

Other groups are also looking to baryonic acoustic oscillations that would provide a standard “ruler” for measuring cosmological distances in the universe. Imagine a super dense region in the primordial plasma of the universe that gravitationally attracts matter. At a certain point the heat created by the interaction of this matter with photons could create a large amount of outward pressure. The gravity pulling inwards and the heat pressure pushing outwards create oscillations, analogous to sound waves, that are used to measure the cosmic distance scale and probe the expansion history of the universe. These sound waves lead to the acoustic oscillations seen in CMB anisotropies, but also leave a faint imprint in the clustering of galaxies and matter today. 

 

There are also proposals for using weak gravitational lensing that is extremely sensitive to the parameters describing dark energy as well as the shape and history of the universe. Teams are also looking to red-shift space distortions due to the peculiar velocities of galaxies that can tell us something about the expansion of the universe. 

 

All in all, today we have a variety of tools to understand the nature of dark energy and we hope to be able to learn something new in the next few years.

 

8) What’s the improvement in precision that you aim to gain from future surveys?

 

The hope is to be able to measure the Equation of State of Dark Energy with 1% precision and the changes of the Equation of State over time with about 10% precision. Achieving this precision will offer a better understanding of whether dark energy is the cosmological constant or perhaps some form of energy temporarily stored in a scalar field that could possibly change over time. 

 

9) Is this one of the topics you are currently involved?

 

Yes, among other things! I am also working on improving the precision of the measurements of the Hubble constant which characterizes the present state and expansion rate of our universe. Refined measurements of Ho could also point to potential discrepancies in the cosmological model. 

 

10) What do we mean by referring to the so-called Hubble constant tension? 

 

The problem is that even when we account for dark energy (factoring in any uncertainties we are aware) we get a discrepancy of about 9% when we compare the predicted expansion rate of the universe based on Cosmic Microwave Background data using the ΛCDM model with the present expansion. The uncertainty in this measurement has now gone below 2% leading to a significance of more than 5σ. New observations by the SH0ES program would likely reduce the overall error on H0 to 1.5%. Moreover, the fact that this result is supported by many independent measurements testifies to its validity.

 

There is something more profound in the disagreement of these two measurements. One measures how fast the universe is expanding today while the other is based on the physics of the early universe - taking into account a specific model - and measuring how fast it should be expanding. If these values don't agree, we may be missing something in our cosmological model that connects the two epochs in the history of our universe. A new feature in the dark sector of the Universe appears in my view increasingly necessary to explain the present difference between the two values.

 

The value of th current expansion rate of the universe, also called the Hubble constant, appears to depend on how it's measured. Observations of the early universe give lower values (gray) than those measured using nearby objects (blue). Studies of red giant stars are giving a value of the Hubble constant that's right in the middle (red). Freedman et al. / Astrophysical Journal.

11) When did the seriousness of the H0 discrepancy become clear? 

 

It is hard to pinpoint a date but I would say it was between the publication of first results from Planck in 2013 and the publication of our 2016 paper that measured the Hubble constant to 3% that it became clear there was a discrepancy.  

 

Since then, the tension has been growing and that’s why it is difficult to define one day for everyone. Various people were convinced along this way as new data came in while there are people who are still not convinced and perhaps never could be.

 

11)  How can this discrepancy be interpreted? 

 

The standard cosmological model (ΛCDM) is more or less the right model that we use to extrapolate the evolution from the Big Bang to the present cosmos. A model with six free parameters that covers a time length of about 13 billion years. This model is based on certain assumptions, that the space in the early universe is flat, that there are three neutrinos, that dark matter is very uninteresting, that the dark energy is similar to the cosmological constant and that there is no more complex physics. 

 

So one, or perhaps a combination, of the above mentioned things can be wrong. Knowing the original content of the Universe and the physics we should be able to measure how the Universe was expanding in the past and what should be its present expansion rate. The fact that there is a discrepancy means that we don’t have the right understanding. 

 

The past decade or so has seen dozens of measurements of the Hubble constant, using sources near (in the box labeled "Late") and far (in the box labeled "Early"). There seems to be a discrepancy depending on whether the measurements are based on the early universe or the present-day universe, as seen in the box labeled "Early vs. Late," though the amount of discrepancy depends on which sources are used. Recent measurements by Freedman and colleagues are labeled "CCHP." Vivien Bonvin / HOLiCOW Team

 

12) Do you have colleagues who are still not convinced about this result? 

 

It is interesting that you ask that as I just came back from a conference in Santa Barbara, California, where this exact question was raised. Almost everybody voted that this was a problem but there were few hands raised expressing alternative ideas. So it seems that this is a serious issue that we have to tackle.

 

In my view, this diversity of opinions is a healthy sign for science. We shouldn’t all think and proceed in the same way. To progress in science we should take into account alternative viewpoints and continuously reassess the evidence we have without taking anything for granted.

 

13) You’ve said that the discrepancy "could not plausibly occur by chance”. What, then, could be its source? 

 

One idea is an episode of dark energy increasing expansion before recombination, another idea is a new relativistic particle and there have been other ideas like a more complicated form of Dark Matter with unexpected characteristics.  It is an interesting and exciting time to explore this question. 

 

We think that the phenomenon that we call inflation is similar to what we call Dark Energy and it is possible that there was another episode in the history of the universe just after the Big Bang and before inflation. There are theories predicting that a form of “early dark energy“ becomes significant just before the recombination epoch giving a boost to the Universe that matches our current observations. 

 

Another option is the presence of dark radiation; a term that could account for a new type of neutrino or for another relativistic particle present in the early history of the Universe and important to define its evolution. The presence of dark radiation would change the estimate of the expansion rate before the recombination period and is also a way to address the Hubble constant problem. Future measurement could tell us if other predictions of this theory are correct or not.

 

14) Does particle physics have a complementary role to play compared to other experiments? Both in terms of research goals but also on other features like big collaborations. 

 

Oh definitely. Both collider and astrophysics experiments could potentially reveal either a property of dark matter or a new relativistic particle or something that will change the cosmological calculations and solve some of the open questions. Particle physics is related to early universe physics and of course it plays a role.

 

In my view there is a certain overlap concerning the contributions of these fields in understanding the physics of the early universe, a lot of cross-talk and blurring of the lines and that’s healthy for deepening our understanding of nature. 

 

Moreover science today is done in really large teams, which means that you have a larger pool of ideas and possible solutions to problems coming in. This poses a challenge as you need to have people coming from a very wide range of backgrounds, cultures, upbringing, education because you really need a broader set of ideas for a scientific collaboration to succeed. 

 

15) Has receiving the Nobel Prize at such a relatively early age being a blessing or a curse?

 

It has been a great honour. You can choose whether you want to do science or not as long as this choice is available. So certainly the Nobel is not a curse. You just have to recognize the freedom of choices that you have. 

 

16) You had mentioned that we live in an exciting period as we can quantitatively answer many questions that were previously left for the Rabbi or the Priest. I am wondering which are the questions today left for the Rabbi? 

 

I think you could still ask them whether there was a sort of creator who made this universe for us to discover these wonders and ask questions about what existed before the Big Bang. These are areas that we presently have limited ability to answer with science. 

 

Of course this can change at any moment. Science doesn’t have a scale or a particular area where it can or can’t work. Curiosity drives us to explore different scales of nature for evidence that could help us understand the universe.

 

17) Which are the future challenges for your research? 

 

Our team is continuing trying to refine the measurements we have been taking while this is a growing community. Hopefully, if you come back in a couple of years we will have more answers to your questions.