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What Illuminated Dark Energy?

Brian Schmidt

Prof Brian Schmidt’s discovery solved three major problems in astrophysics. Credit: Belinda Pratten

By Tamara Davis

Science rarely overturns existing paradigms, so why was the astonishing announcement that a mysterious “dark energy” was accelerating the expansion of the universe so quickly accepted by cosmologists?

Reporting of scientific discoveries usually emphasises the surprises, the awe, the overturning of previous “knowledge”, and the whiz kids showing that commonly held wisdom is wrong. That’s no wonder, because the plodding progress of incremental improvements with frequent missteps but steady growth hardly makes exciting news, even though it’s the more common route to success. In science, just as in music, the “overnight success” of enormous breakthroughs pretty much always rides on the back of years of training and hard work.

That’s certainly true in astrophysics, and certainly true for “dark energy” research, which is my specialty. Dark energy is the name we give to the unknown cause of the acceleration of the expansion of the universe. The 1998 discovery that the expansion of the universe was speeding up, when everything we thought we knew about gravity told us that it should be slowing down, was definitely a surprise and rightfully evokes awe. It looks as though dark energy, whatever it is, makes up about 73% of the universe, and discovering that was certainly a shock to the researchers involved.

It seems an outrageous claim, really. We discover an unexpected result, give it a name beginning with “dark”, and claim to have found that most of the energy in the universe comes in that form and has some sort of antigravity. Nuts.

However nuts it may seem from the outside, though, that discovery won last year’s Nobel Prize for Physics. Again, most of the reporting about the prize emphasises the shocking nature of the discovery and how it will revolutionise physics. That’s true.

However, by emphasising the astonishing nature of this result, and the excitement and mystery about the nature of dark energy, we fail to convey how rock-solid a result this is, how it solves numerous outstanding problems in astrophysics, and how it has now been confirmed to truly impressive precision by numerous other completely independent observations using completely different techniques.

How is it that such an outrageous result was accepted so quickly, confirmed so thoroughly, and passed from speculation to fact so convincingly?

Cast your mind back to 1997, before dark energy was discovered. Astrophysics was in a bit of a pickle. Stellar astrophysicists had very good models of stars and could tell that the oldest stars were about 13 or 14 billion years old. Meanwhile, measurements of the expansion of the universe put the age of the universe at a mere 9 or 10 billion years. Trouble: the universe seemed younger than the stars it contained.

That’s not all. Measurements of the density of matter were getting more and more accurate, and the average density fell about 70% short of what was needed to make the universe flat. (The curvature of space depends on density. For example, space is highly curved around a black hole.)

This wouldn’t be a problem if it weren’t for the fact that the theory of inflation, which we believe happened in the very early universe, predicts very strongly that the universe should be flat. Either inflation was wrong or the observations were dodgy. Arguments ensued.

Finally, counts had been made of the number of galaxies in regions of space nearby and far away. There were too many galaxies in the regions far away, so the maligned observers were again disparaged for getting something wrong. Most commonly it was thought: “Oh well, galaxies evolve and merge, that must be the explanation”.

Despite their claims that galactic mergers could never be efficient enough to explain the effect, the observers were generally not believed. Interestingly, the observers did note that if the universe was accelerating then the volumes would be bigger than they had accounted for, and the number of galaxies would be right.

That’s three major problems. Age. Density. Number of galaxies. The discovery of the acceleration of the universe solved all three.

A universe that accelerated to its current rate of expansion is older than one that only ever decelerated. Do the calculations and you find that for the amount of dark energy they found, the universe should be about 13.7 billion years old, which nicely matches the age of the oldest stars.

Meanwhile, the reason that we had a 70% shortfall in the density needed to make the universe flat is that we were only considering matter. The 30% we could find contains both normal and dark matter, but we had neglected the density of the dark energy. More accurate observations place the energy density in matter at about 27%, and that of dark energy at about 73%. After adding the observed dark energy, observations and theory both agree that the universe is flat.

Finally we can say that the number count people were right all along, and we should have listened.

So you could say that the ground was fertile for the discovery of acceleration. Its discovery let astrophysicists breathe a sigh of relief, shake hands with their adversaries because everyone had been right, and accept that the acceleration was the missing piece that let the rest of the puzzle fall into place.

But there is also one final reason that the discovery was accepted so quickly, and this reason is partly sociological. There were two competing teams doing the observations. They both came up with the same result at about the same time using completely different analysis pipelines and different sets of data. There was very little chance that both could have made the same mistake and come up with the same incorrect answer. Scientists are human. It’s easy to dismiss one unexpected result (apologies to the galaxy counters), but two starts to look like a pattern.

Since that original discovery, many other types of data have confirmed the acceleration to stunning accuracy.

I could talk about the cosmic microwave background, about the Bullet Cluster, about time dilation, about gravitational radiation from inspiralling pulsars, about active galaxies, about large-scale structure, and about watching that structure grow. The advent of digital cameras on large precision telescopes, combined with supercomputer analysis, has revolutionised the kinds of questions we can address using astrophysics and the precision with which we can answer them. We’re asking the same fundamental physics questions that they are trying to answer at particle accelerators such as the Large Hadron Collider. With astrophysics we’re just using the natural experiments the universe puts on for us for free.

What sticks with me when I consider all these multivaried aspects of cosmology is the amazing efficacy of physics. What we have learned is astonishing, and leaves plenty of exciting mystery for the next generation of astrophysicists and theoreticians to explore.

The truly amazing thing is how detailed and tightly knit our understanding of the universe has become. The data we have concerning the universe these days is phenomenal. We can see light from times before stars and galaxies even existed, back when the universe itself was so hot and dense that the whole thing was glowing like a star. Using astrophysics we can now find things out about the very fabric of space–time, the way fundamental physics works, and perhaps one day discover how to turn these into new technologies, energies and human capabilities here on Earth.

The next generation of telescopes will bring large all-sky surveys to the fore, with enormous amounts of data looking back close to the dawn of time.

It is an exciting time to be an astronomer!

Tamara Davis is a Research Fellow with the University of Queensland’s School of Mathematics and Physics.