Most of the world is dark – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Namely Names > 2017 > Most of the world is dark

20 January 2017

Most of the world is dark

research award

- Jens Hjorth, astronomer and recipient of Denmark’s largest individual research award - the VILLUM FONDEN’s 2017 Annual Award - for breakthroughs in the field of cosmology.

Professor Jens Hjorth, Dark Cosmology Centre (DARK)

"The universe is dark – and mainly consists of dark matter and dark energy. Only 5% of the universe is composed of luminous matter from galaxies, planets and stars. Yet little is known about the rest of the universe. But we do know that it is there, because dark matter interacts with gravitational forces," according to Professor Jens Hjorth.

The big questions that we all ask

Professor Jens Hjorth is the director of the Dark Cosmology Centre (DARK) at the University of Copenhagen’s Niels Bohr Institute. He established the research centre in a quest for answers:

- "I like to ask the big questions. Who hasn’t sat outside, gazed up at the sky and wondered: ‘what is out there?’ It is a primal fascination, one that does not require being a scientist to experience. I'm totally driven by this curiosity. An epistemological, philosophical need for perspective on our existence. I want to know how things fit together. To qualify what we know, and what we do not – and I am unsatisfied until I get answers," he says.

Dark matter - hunting for the great forces behind structure in the universe

Jens Hjorth explains that the majority of things in this world are dark, not because of any dark color, but because they are invisible.  The immense dark universe does not interact with light. Nor does it emit or absorb light. As such, most of the universe is cut off from light.

- "We talk a lot about reason and experience. ‘I've seen it with my own eyes’, we say. And thus, we believe. When we can't see something, we do not believe that it exists. This won’t get us far if we intend to understand reality", smiles Hjorth, who continues with his ambitions for DARK:

Professor Jens Hjorth

"Originally, one of our ambitions with DARK was to determine what dark matter is. Today, we still don’t know. But we do know more and more about how it behaves, and we cannot rule out weak interactions. One possibility is that it interacts with itself. I think that it is a type of particle, and that it is likely that we will be able to detect it one day.

One thing that amazes us, is why there is so much of it. Most of the universe is filled by dark matter. But we don’t know its form. We would also like to discover its properties, besides its ability to interact with gravity. In a way, neutrinos qualify as a type of dark matter, because they interact very weakly. But they weigh too little", says Jens Hjorth.

Researchers around the world are working diligently upon "dark matter" detection projects to develop advanced detectors that are able to identify dark matter:

"But we are all shooting into the dark somewhat, because we don’t know what it will interact with. The physics of luminous matter is more tangible because it interacts with light."

One thing researchers do know is that dark matter is all significant for how the universe is structured. Dark matter dynamics determines the appearance of the universe and the size of its galaxies.

Dark energy accelerates the universe

- "We call it dark energy, because it has properties that relate to energy. Dark energy causes the universe to accelerate and expand exponentially. Therefore, we live at a moment in the history of the universe during which the universe is expanding more rapidly than ever before.  Everything is moving apart. Galaxies are moving away from us, and we moving away from them. As the universe expands, the energy density of objects like Earth is relaxed, while the energy density of dark energy remains constant. As such, dark energy will become more and more prevalent and dominant in the future. This builds upon a mix of observations and an assumption that Einstein's Theory of General Relativity remains true.  However, it is possible that we have incorrectly described the universe.”

Within the field of astrophysics, the possibility of a ´dark sector´ is discussed. In it, dark matter in this universe has its own chemistry, stars and dark galaxies without luminous matter. A unique world which doesn’t just include anonymous single particles, but which exists parallel to our own world.

- "We have looked for large dark clumps without luminous matter. We have found systems with very little luminous matter, but we have not detected any systems composed entirely of dark matter. Dark matter has a few properties that, among other things, make it difficult to condense easily, thus making it more difficult to form high density clumps that resemble anything that we are used to,” says Jens Hjorth.

Gamma-Ray Bursts 

One of the reasons that Jens Hjorth is acknowledged as among the leading cosmology researchers in the world, and that he has received the VILLUM FONDEN's Annual Award, is for his work on short and long gamma-ray bursts. With the help of Earth-based satellites and telescopes, Hjorth has documented that long gamma-ray bursts occur when a massive star, known as a super nova, explodes very far away. The discovery has catalyzed a paradigm shift in the field. Jens Hjorth was research director during the first discovery of visible light from a so-called gamma-ray burst, a burst that  lasted for less than two seconds.  Detection has shown that the origin of short gamma-ray bursts is different from the origins of long gamma-ray bursts, and that they are probably due to the merging of extremely compact stars.

"I have been involved in shedding new light on what gamma-ray bursts are. I have also helped locate the so-called ‘optical afterglow’ of a short gamma-ray burst," says Jens Hjorth.

In 2003 and 2005, research into short and long gamma-ray bursts was deemed to be among the top 10 international science breakthroughs in the journal Science’s Breakthrough of the Year.

The future, gold and exploding stars

Jens Hjorth will soon commence research on the origins of solid silicon and carbon-based substances. Substances that our world consists of, and that were formed very early on in the formation of our universe. He will also develop more knowledge about how the heaviest elements were formed, e.g. gold.

- "One hypothesis that I would like to continue to work with is that gold is formed by two compact neutron stars which weigh the same as our sun. They emit short-duration gamma-ray bursts, and possibly, neutron-rich materials such as gold,” says Jens Hjorth, who continues: “Hopefully, we will simultaneously be able to locate the source of any gravitational waves with precision. When two neutron stars rotate around one another, they emit gravitational waves prior to their merging."

Jens Hjorth and his research team will continue working to identify the dynamic universe.

- "Pictures are only static. When we look at the sky using the technology that has become available to us over the past few years, we are able to ascertain dynamics and variables - everything that occurs suddenly, and then disappears. For example, an exploding star.”

He adds:

- "We can describe creation. In fact, I feel that, as a scientist, I have moved into the domains of others, such as theology. We are asking the same types of questions."

In addition to his research and managerial duties at DARK, Jens Hjorth continues to spend time educating astronomers and physicists, for whom there is a great demand on the labour market, as in the fields of banking and IT.

"I'm an astronomer in order to expand our awareness and break new boundaries. We cannot anticipate what lies ahead. A new world view perhaps?