(Image: Quibe)
A FEW years ago, I was interviewing a cosmologist applying for a fellowship to study the cosmic microwave background. I asked her what she thought the great developments in the field would be over the next few years. Her answer: pretty much none; the big picture was more or less done and dusted.
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It might seem like she was shooting herself in the foot. The accidental discovery 50 years ago of the CMB, the afterglow of the big bang, is perhaps the greatest triumph of our general-relativistic model of the universe. It was detected as an unexplained hiss in an antenna built for experiments on terrestrial microwave communication. Since then, studies of the CMB have provided convincing proof that our universe began in a hot, dense pinprick and has been expanding ever since, releasing this radiation when it had cooled sufficiently for the first atoms to form.
Studying the CMB has allowed us to characterise the universe’s beginnings at energy scales unreachable by CERN’s Large Hadron Collider particle accelerator, or any conceivable successor. Ground-based experiments, and latterly space missions such as the and satellites, have used the radiation to measure the geometry of the universe with incredible precision and provide our best figure yet for its age – 13.8 billion years. They have also elucidated the astounding facts, first flagged in studies of galactic rotation and far-off supernovae, that the vast majority of cosmic stuff comes in forms that we cannot see: dark matter and dark energy.
Yet I liked the candidate’s confidence, and she got the job. Although we expect to increase the accuracy of the measurements already made, and the mysteries of dark matter and energy remain, the big picture of the general-relativistic universe has indeed been fleshed out. Too many observations agree too well for it all to be a house of cards.
Comprehensive surveys of how galaxies are distributed that are now in the works will help fill in the gaps and shed light on how dark matter and energy have influenced the universe’s evolution. But there is much detail still to be gleaned from the CMB itself. Our most compelling description of the early universe says it underwent a period of accelerated expansion, known as inflation, that stretched microscopic quantum fluctuations of space-time out to astronomical scales. According to the precepts of general relativity, these events should have sent ripples out through space-time. Early in 2014, CMB measurements by the BICEP2 experiment at the South Pole seemed to have found these primordial gravitational waves – although under closer scrutiny, it turned out that the observed effect was caused by effects within our own galaxy. The search for gravitational waves continues in the CMB and elsewhere (see “General relativity at 100: The missing piece of the jigsaw“).
Up till now, we have pored over the CMB rather as we would a picture of a distant ancestor, trying to discern the traits in it that led to what we see today. But it can also be a backlight to illuminate the present or, at least in cosmological terms, the very recent past – and so show up the subtleties of how gravity has shaped our universe.
“If general relativity were ever proved wrong that would be a true revolution”
Take the gravitational lensing of CMB light as it propagates towards us – the effect that Arthur Eddington used to provide the first proof of general relativity in 1919 (see “100 years of general relativity”, above). CMB photons will be deflected by warps and folds in space-time caused by the large-scale distribution of matter in the universe, ever so slightly distorting our view. By tracking this effect over time, lensing measurements from the Planck satellite have confirmed that the universe’s expansion is indeed accelerating under the influence of dark energy.
Still more sensitive measurements of the distorted CMB should allow us to work backwards to the distribution of dark matter, which apparently makes up more than 80 per cent of every galaxy. This gives us a new window on how the complex filaments, walls, clusters and voids of the “cosmic web” have formed over time, without having to worry about the messy details of normal matter’s interactions (see diagram).FIG-mg30420701.jpg
The biggest test
Distortions introduced when photons from the CMB scatter off electrons in intervening galaxy clusters will also allow us to measure how fast these clusters are moving around, and how quickly they are collapsing gravitationally, sucked into denser forms through dark matter’s influence. That gives us a new way of testing general relativity’s predictions. For although the theory has been exquisitely studied on the scale of the solar system and in the orbits of neutron stars, it has yet to be tested on scales spanning billions of light years.
The safe bet is that general relativity correctly describes the universe out to cosmological scales, for all that we are baffled by the dark spectres it calls into life. If general relativity were ever proved wrong that would be a true revolution. It would call into question the existence of dark energy as the driving force behind the universe’s expansion in recent eras. But it would also force us to figure out what hallowed principles we need to jettison to obtain a description of gravity on cosmic scales that is different to the one we have been using for the past century. That is a question to which few people as yet have wagered an answer.
Read: “General relativity at 100: Einstein’s unfinished masterpiece“
This article appeared in print under the headline “General relativity at 100: Einstein’s witness in the sky”
