"Damn, I made a mistake! How inopportune, I have to fix this before others realize I was wrong. That's the first thing I thought when I saw the result," recalls astrophysicist Adam Riess, from Johns Hopkins University (USA). For that result, which in the end turned out to be correct, and which implied that the Universe is expanding faster and faster, and therefore that there must be a dark energy opposing gravity, Riess won the Nobel Prize at only 41 years old.
The scientist is in Catalonia on the occasion of the twentieth anniversary of the Institute of Cosmos Sciences of the University of Barcelona (ICCUB). Over the next six years it will collaborate with the ICCUB on a project to specify the expansion speed of the Universe. Funded with 12 million euros by the EU, the project is coordinated by ICCUB researcher Licia Verde.
Did you try to find the error?
Clear! I was in my second year of my PhD. I knew almost nothing about astrophysics. But I knew that the result the computer gave me didn't make sense.
What result did it give you?
That the Universe has a negative mass, which is physically absurd. Computers have no idea about physics, or at least they didn't then.
How did you find the error?
I reviewed everything I had done, absolutely everything. But I wasn't able to find it. The only way to eliminate negative mass would have been to introduce a force contrary to gravity. In the end I asked some colleagues for help. I sent them all the data so they could find out what was wrong. Among them Brian Schmidt, who shared the Nobel Prize with me.
Did they find anything?
Brian replied, “Hello, lambda.” He had seen the same thing as me.
Lambda?
It is Einstein's cosmological constant. He introduced it into his equations to counteract the effect of gravity [because Einstein believed that the Universe was static, and without lambda it would have contracted]. But later Einstein himself disowned it and said that it had been his biggest mistake.
Did you think it was a good idea to use lambda?
Looking at the data, it seemed so. But when I said "with lambda it might work", others told me "are you kidding? What you're saying is nonsense."
But he kept it.
I was not aware of what was at stake and I had not stopped to think about the consequences. He was young and stupid, and not very wise. I think that young researchers are more flexible when it comes to trying new things than older colleagues who are sometimes more reticent.
In the end, was the problem solved with lambda?
But not immediately. He wasn't sure if what the data said reflected the reality of the Universe or was a mirage.
Why should it have been a mirage?
Because a mirage is something we see but it is not there. The image of the mirage is real but reality does not correspond to the image. This can also happen to us when making scientific observations. What we were proposing was so extraordinary that we had to be very sure that the image did not deceive us.
How long did it take you to be convinced that the accelerated expansion of the Universe is real and that there must be dark energy?
I stopped having doubts when other teams, interrogating the Universe with methods different from ours, reached the same conclusion. It was around 2005, almost ten years after our observations.
How does it feel to have corrected Einstein?
I think Einstein would be amazed by the accelerated expansion of the Universe.
Based on your experience, what advice would you give to young researchers who find unexpected results?
The first thing is that you have to be skeptical with your data. As Richard Feynman said, you have to be careful not to fool yourself because you are the easiest person to fool. Once you are sure that your data is good, you have to be willing to say what you see.
Does it help to have certain character traits to defend your data when it goes against what the rest of the world believes?
Absolutely. In science, first, curiosity. If you find something that doesn't add up, settling for saying “I don't know” is not an option. If you want to achieve complete understanding, curiosity must be your guide. Also important is the conviction that you are questioning nature and that you have to be open to listening to what it tells you. Personally, I have more confidence in what I see in the sky than what theory can say, because theory is based on incomplete knowledge, while the sky shows you reality.
What captivated you about the Universe?
The amazement. Before my PhD I loved the beauty of physics. Then I discovered that in space the rigor of physics and mathematics can address truly amazing questions. I still find this combination of scientific rigor and wonder fascinating.
Is there a mystical component?
For me, yes, you can say it like that.
What enigmas of the Universe intrigue you now?
There are several, they may be related to each other. What is the nature of dark energy. What is the correct theory of gravity. Why, when we question the expansion of the Universe with different techniques, we obtain different answers. What happened before the big bang...
Before the big bang? But didn't time start with the big bang?
Don't know. And I don't know how we could study it either.
As an experimentalist, do you think there may be a way to find out if our Universe is one among many?
If we define the Universe as everything that we can observe, talking about other universes does not seem very useful to me. This is what is called the multiverse, which leads to theories that cannot be verified experimentally. For me it is certainly not science, it is speculation.
