In the year 1909, a little-known astronomer named Vesto Slipher began a series of painstaking observations at the Lowell Observatory in Flagstaff, Arizona, in the US. The observatory had been built primarily to look for evidence of Martian canals, but Slipher had set his sights well beyond the Red Planet and its putative inhabitants. His interest was in the nature of fuzzy patches of light called nebulae. Were they gas clouds in the Milky Way, or far-flung galaxies in their own right? Slipher carefully measured the colour quality of the glowing patches, and found that the fainter they were, the redder their light. Through the lens of history, we can now see that this discovery marked the beginning of cosmology as a proper science.
Slipher’s “red shift” came to the attention of astronomer Edwin Hubble, who understood that the reddening effect implied that the objects were rushing away from us at great speed. Using a more powerful telescope, he confirmed that most of the nebulae were in fact distant galaxies. On 23 November 1924, Hubble announced in the New York Times that the entire universe is expanding. It was one of the most momentous scientific pronouncements of all time.
It took several more decades, however, before the modern big bang theory became established, according to which the universe was propelled on its path of expansion from an explosive origin 13.8 billion years ago. The intense heat of the primordial explosion still exists as a fading afterglow, filling all space with a sea of microwaves. This cosmic microwave background, or CMB, was detected by accident in 1967 by two radio engineers. It was immediately apparent that this was the big bang’s smoking gun, and that, etched into the structure of the CMB, lay vital clues about the origin and nature of the universe.
In November 1989, NASA launched the satellite COBE (Cosmic Background Explorer) to map the remnant primordial heat in detail. A few weeks later, NASA released the first heat map of the universe – a colour-coded palette of amorphous splodges indicating slightly hotter and colder patches of the sky. The golden age of cosmology had begun.
Over the three decades since, the CMB has been data-mined to enormous precision, first using COBE’s results, then those of other instruments, the most recent of which is the European Space Agency’s Planck satellite. Piecing together the CMB observations with those from powerful ground-based telescopes, astronomers and physicists have been able to construct the Great Story of the Universe from the first split second to today, in extraordinary detail. During my career, cosmology has gone from being a speculative backwater to a precision science.
In spite of this ringing success, some ugly cracks have started to appear in the cosmic facade. If theorists are to be believed, those tell-tale splodges in the CMB carry a faint echo of what the universe was doing a mere billion-trillion-trillionth of a second after the big bang, an era known as the inflationary phase, when the universe abruptly leapt in size by an enormous factor, as if it had taken a sudden deep breath. Quantum effects during inflation imprinted slight fluctuations in density and temperature on the nascent cosmos, sowing the seeds of what was to eventually evolve into the large-scale structure of the universe – galaxies and clusters of galaxies. The splodges in the CMB are evidently fossils from the edge of time itself, writ large and frozen in the sky.
The laws of quantum physics neatly explain the characteristic patterning observed by COBE and its successors, but there are a couple of discrepancies. The most glaring concerns a large patch in the Southern Hemisphere constellation of Eridanus which is weirdly much cooler than it should be based on statistical fluctuations. It looks like something has taken a giant bite out of the universe, leaving a supervoid. The Eridanus cold patch has led to some imaginative speculation. Could it be a blemish left by another universe bumping into our own? Might it be a portal into a region beyond the known universe? Or some sort of matter-destroying “bubble”?
Another fly in the cosmic ointment concerns the rate that the universe is expanding, known as Hubble’s constant. For decades astronomers sharply disagreed with the measurements, until a few years ago they agreed on a compromise value. Just as the dust was settling on this vexed issue, a new way to measure Hubble’s constant, using the splodges in the CMB, gave an answer seriously out of whack – about 10% smaller than the agreed number. Because the inferred age of the universe hinges on the value of Hubble’s constant, the implication is that 13.8 billion years is now an underestimate.
Dark matters of dispute
Next on the list of unanswered questions is the nature of dark matter and dark energy. Astronomers are certain that the stuff of which you, me and the stars are made is but a tiny percentage of all there is.
Fully five times as much matter is in some unknown form that doesn’t seem to interact noticeably with normal matter, except for the gravitational tug it exerts. The smart money is on some sort of weakly interacting heavy subatomic particle, legions of which must be passing through us all the time without causing a shudder. The race is on to try to detect the occasional fleeting passage of a dark matter particle, or perhaps to create one in giant accelerator machines like the Large Hadron Collider at CERN in Switzerland.
Even if a dark matter particle is nailed in the near future, it still leaves unanswered the nature of the stuff that makes up three-quarters of the mass of the universe, the thing known as dark energy. It is not really matter in the normal sense of the word. Rather, the best way to envisage dark energy is the energy of empty space (which is why we can’t see it).
The idea that space itself might have energy goes back to 1917, when Einstein realised that if the energy of space isn’t strictly zero then space would be self-repulsive; that is, it would possess an intrinsic propensity to expand, faster and faster. In effect, space energy is a form of anti-gravity. It wasn’t taken seriously until the 1990s when, low and behold, astronomers (including Brian Schmidt, now the Vice-Chancellor of ANU) found that the expansion of the universe is speeding up. Dark energy would do the trick nicely.
Not everyone is happy about invoking Einstein’s anti-gravity to explain the accelerating expansion. Part of the problem is that the amount of energy in, say, a million cubic kilometres of empty space is entirely arbitrary. It is, however, an exceedingly tiny number: astronomers measure it to just enough energy to boil a kettle if it could be harnessed. But why that particular number and not some other?
Appeals to quantum physics to derive a value for dark energy fail spectacularly. One estimate is out by about 120 powers of ten! Maybe space is permeated by a new sort of field that produces just the right amount of cosmic repulsion, but so far all we have to show is a lot of different models and calculations and nothing definitive.
The question “What is dark energy?” is high on the list of outstanding problems in fundamental science.
This excerpt is republished online from Cosmos Magazine issue 92, which went on sale on Thursday 2 September 2021.
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Paul Davies is Regents' Professor and Director of the Beyond Centre for Fundamental Concepts in Science at Arizona State University. He is also a prolific author, and Cosmos columnist.
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