The Dark and Black Recesses of the Universe

Some 411 years ago, Galileo Galilei spotted three small ‘stars’ circling Jupiter with one of the world’s first telescopes, revolutionising understanding of the cosmos and our place within it. Ever since, astronomy has relied on telescopes and similar technologies to capture visible light and other electromagnetic radiation emanating from the universe.

But this poses a problem if we want to understand the parts of the universe that seem almost allergic to light. How do we ‘see’ a black hole when its staggering gravitational pull is so strong that no light can escape? Similarly, how can we unmask the nature of dark matter – thought to make up roughly 25 percent of the universe – when it does not absorb, reflect or emit light? As for dark energy, which appears to be roughly 70 percent of the universe, all we know for certain is that it is causing the rate of expansion of the universe to increase. How can we learn more?

These were the deep questions at the heart of a panel discussion on day 4 of #LINO70. Primed to offer their thoughts on the ‘dark universe’ were young scientist Saskia Plura as well as Nobel Laureates Reinhard Genzel, David Gross, Saul Perlmutter and Rainer Weiss. 

Signals From the Void

Genzel was first to take the stage. By observing stars in the area around Sagittarius A* at the heart of the Milky Way, Genzel indirectly observed the gravitational pull from an invisible massive object. For this, he was awarded a share of the 2020 Nobel Prize in Physics “for the discovery of a supermassive compact object at the centre of our galaxy”.

Moderator Sybille Anderl was quick to point out that the Nobel announcement made no mention of the object being a ‘black hole’. So, is it really a supermassive black hole? “While it’s highly plausible that what we have seen is indeed a black hole, there are still some loopholes,” explained Genzel.

To provide incontrovertible proof, researchers will need to measure the object’s spin and to check if it conforms to the no-hair theorem, meaning that it can be completely characterised by its mass, electric charge and angular momentum. Though Genzel clarified: “That’s a very, very challenging experiment”.

The announcement of Weiss’ 2017 Nobel Prize in Physics made no mention of black holes either, but instead focussed on what he, Barry Barish and Kip Thorne as key members of the LIGO Collaboration had achieved in detecting the first gravitational waves.

Like the Nobel Committee, at the time of the discovery, Weiss too was cautious to claim that what LIGO had picked up was indeed the faint ripples in spacetime caused by the violent collision of two distant black holes. “I was very sceptical,” he recalled. “We had just made major changes [to LIGO] and almost within a day that we opened the apparatus up and started looking, we saw this signal.” But with the detection of additional and different signals later in 2015, “everyone began to believe that it wasn’t a fake”.

Cosmic Discrepancy

Gravitational wave astronomy was born, opening an entirely new window to the universe. Might this new window offer a view into the nature of other dark elements of the universe besides black holes? For example dark energy? One way to do so would be providing a new method to measure the Hubble constant, which is the yardstick for the universe’s expansion rate today. Currently, there is a discrepancy in the values derived by the two main techniques to calculate it, commonly called the ‘Hubble tension’. “If they don’t [agree], that has a severe impact on our understanding of dark energy pressure, and it is an enormous challenge to the Standard Model of Cosmology,” said Gross.

Perlmutter – who received the 2011 Nobel Prize in Physics with Brian Schmidt and Adam Riess for discovering the accelerating expansion of the universe – stood ready to explain this Hubble tension and its relevance to dark energy; which, in his words, is a “placeholder name” for whatever is driving the universe’s accelerated expansion. He went on to summarise the latest research on dark energy in simple terms, and later further expanded on this with Riess in their Agora Talk.

Perlmutter and Riess clarified in their talk that the challenge in exposing dark energy’s nature is not a lack of ideas. “At one point, we were getting papers proposing new ideas for dark energy at the rate of one published every 24 hours over 10 – 15 years,” said Perlmutter. The real difficulty is in ensuring these ideas line up with all the observations that have gone before – something only the Standard Model of Cosmology comes close to doing.

“One of the things that’s so fun about cosmology is that it’s sort of like a sweater that’s well made, but there’s a few loose threads, and it’s always possible you pull on a loose thread and the whole sweater unravels,” said Riess. “But it’s much more likely these are just small loose threads – they aren’t necessarily a licence to throw away the whole sweater.”

Demystifying Dark Matter

While deciphering the nature of dark energy appears to be at an embryonic stage, exposing dark matter’s identity is a more palpable prospect. For instance, just three months ago, researchers working on the Muon g-2 particle physics experiment at Fermilab confirmed an existing anomaly and announced a new one in the magnetic properties of the muon. “All these anomalies can surely help us find new physics beyond the Standard Model [of Particle Physics], be it dark matter or something else,” suggested young scientist Plura.

Most encouraging for Gross is that scientists have more to go on with dark matter than dark energy: “Dark matter has been observed gravitationally by our colleagues in astronomy – the evidence is overwhelming,” he enthused. “But what is it made out of, what are the constituents that make up dark matter? This has been an ongoing problem.”

As a result, both Gross and Plura were keen to encourage more young scientists to explore and experiment with different ideas for dark matter. “There are so many new models of dark matter waiting to be tested,” said Plura. “It’s an exciting time to be working on this.”