The Week in Space and Physics: The Mysteries of an Expanding Universe
On new observations of dark energy, NASA's newest space telescope, the furthest known galaxy, and a partial solar eclipse.
From the outside, it looks much like any of the other domed telescopes scattered around Kitt Peak in Arizona. Indeed, in many ways it is like those other telescopes. Night after night the Mayall Telescope surveys the sky, captures photons coming from unimaginable distances, and feeds them into a host of waiting instruments.
Yet it is here, cosmologists said last week, that one of the biggest discoveries of the last quarter century of physics may have just been made. For inside the Mayall Telescope is an instrument called DESI, and for the past four years it has been scouring the universe for traces of dark energy. Now, if those cosmologists are right, it may be on the verge of forever transforming our view of that mysterious substance.
The initial discovery of dark energy came back in 1998, when cosmologists stumbled across a strange acceleration in the expansion of the universe. This was unexpected. On the large scale, as best we could see, the universe was dominated by gravity. It is, after all, the force that holds galaxies together, sculpts them into clusters, and should ultimately pull everything back together.
Yet the measurements in 1998 showed the opposite. Something else, they implied, was acting on the largest of scales; some unknown repulsive force powerful enough to overcome the gravitational attraction of every planet, star, and galaxy in the universe.
Physics had, and still has, no good explanation for what this could be. Cosmologists thus dubbed it dark energy, made some basic assumptions about how it must behave, and then worked it into their standard models of how the universe formed and evolved over time. This has, in many ways, worked out. Although we don’t know what dark energy is, computer models that incorporate it have managed to simulate universes that look much like our own.
That is, until now. Last week, the scientists of the DESI project released a detailed analysis of their first three years of observations. In it, they said, are clear signs that dark energy is weakening over time, a discovery that threatens to shatter our cosy assumptions about its nature. It was, as several headlines put it, a result both shocking and exciting.
For our current models of cosmology this is, of course, bad news. They were built around an assumption that dark energy has remained unchanged since the Big Bang. If that is wrong, it means we will need to rethink both our understanding of the early universe and of its ultimate fate.
For cosmologists, however, this is an exciting moment. If the old models must be overthrown, then the race will be soon be on to find a replacement. That will mean once again questioning the basic assumptions of cosmology and rethinking the theories we once held sacred. Such opportunities to reshape the cosmos are rare indeed.
The Signature of Inflation
On Tuesday last week, a rocket carrying NASA’s new SPHEREx observatory launched from Vandenberg. Over the next two years the orbiting telescope should create four full scans of the celestial sphere, each charting the positions of hundreds of millions of galaxies.
SPHEREx will do this with the help of a spectrophotometer, a device that measures how the intensity of light varies with wavelength. The observatory will thus image the celestial sphere in close to one hundred different “colours”. Each of those colours will correspond to a different band of infrared light
This, NASA says, will ultimately be used to produce a three dimensional map of how galaxies are scattered in space. Researchers then hope to probe that map for clues about the initial inflation of the universe, a mysterious process thought to have taken place in the first millionth of a second after the Big Bang.
In that brief moment, modern cosmology posits, the universe blew up from something no more than a metre or so across to a vast expanse that could only be measured in light years. The reasons why this happened are sketchy, but the idea that it did does solve a lot of difficult questions about why our universe looks like it does.
SPHEREx is unlikely to tell us why this inflation happened. But its measurements should still let us pin down some of the parameters of that initial expansion, and thus give us a bit more data to go on. In time, indeed, that might help us reformulate the theory and so put it on a surer footing.
Alongside that task, SPHEREx will hunt for signs of water ice in our own galaxy. Of particular interest here are molecular clouds: regions of gas, dust, and ice within which the process of star birth can begin. Astronomers believe they should contain plenty of water. Yet many questions about them remain – we don’t know, for example, how ices gather into clouds, what form they take inside them, and what happens to them as stars and planets form. SPHEREx should offer us some answers.
Early Galaxies Continue to Surprise
The Big Bang, according to modern measurements, took place almost fourteen billion years ago. The first atoms formed about four hundred thousand years later, and then, after another two hundred million years or so, the first stars began to shine.
We have not seen the light of those stars. But last year the James Webb telescope picked up photons created when the universe was just three hundred million years. They were coming from the poetically named JADES-GS-z14-0 galaxy, the most distant of all the galaxies ever seen by human eyes.
Those photons did not look like astronomers had expected. Models had suggested galaxies would take time to form, and that the earliest of them would be small and pure – containing mostly hydrogen and helium, rather than heavier atoms like oxygen and carbon. Yet JADES-GS-z14-0 is big and bright, streaked with dust, and, according to a recent study, has a surprising amount of oxygen.
That all implies the galaxy formed fast, coming together about as quickly as could be possible in the early universe. It must already have seen several generations of stars come and go, with the demise of each adding heavier elements to its mix. And none of that fits with our current models and ideas about the early years of the universe.
Of course, this galaxy is an outlier. We have only seen it because it is bright and unusual. Indeed, there are probably countless other fainter galaxies of the same age that we have missed. That, of course, means we should be cautious in drawing conclusions from it. Yet the fact it exists at all is astonishing, and plenty of astronomers will thus use it as a starting point when rethinking our ideas about how the first galaxies came to be.
When The Crescent Sun Rises
A partial solar eclipse will take place this week, tracing a route over the Arctic and Northern Atlantic on March 29. The eclipse will reach its maximum extent over Nunavik in northern Quebec. At that point, around ninety percent of the Sun will be covered by the Moon.
The eclipse will also be visible – albeit with less of the Sun covered – across north-western Africa, Europe, and along the eastern coast of North America. Those who rise early enough on the Eastern coast, and especially those north of Boston, will see the eclipse at sunrise - and may thus catch the spectacular sight of a rising crescent Sun.
The only other eclipse this year will take place over the Pacific Ocean in September. Like this one, it will be a partial eclipse, and, if you live on the eastern coast of Australia, will take place as the sun rises.
