Black holes

Violent gravity
Stars act as element factories out in the universe. Here, lighter elements combine to form heavier ones. The violent explosion leaves behind a very heavy remnant - a neutron star; and if it's really heavy - a black hole.

Black holes are one of the things you can learn about in the Cosmos exhibition.

Black holes are not holes as we know them. They are a collection of material that is so tightly compressed that gravity is enormous. So big that nothing can escape from it. Not even light. Because light cannot escape the black hole, we cannot see it. The opposite of light is darkness, and darkness in the universe is black, which is why it got that name.

The word hole is particularly misleading as it is not a real hole in the three dimensions that we can perceive. We call it a hole in spacetime, but spacetime has time as a fourth dimension, which is why our brain may have difficulty understanding it. It was Albert Einstein who first predicted that black holes should exist in his ‘general theory of relativity’ and the name has stuck.

Black holes are one of the most violent phenomena in the universe and anything that gets too close will be caught by gravity. Once inside the black hole, there is no chance of escape.

So not even light can escape a black hole. On the other hand, black holes are thought to be responsible for some of the most luminous phenomena in the universe. This happens when material around a black hole that is falling towards the black hole is heated to several thousand degrees and begins to glow. This is called a quasar and we see them at the centre of many galaxies.
 

Several types of black holes
Black holes come in many sizes and with different properties. Black holes can probably form in many different ways, and not all of them are equally well understood by scientists. But some black holes are probably formed when large stars die.

When stars weighing more than 8 times our sun die, the interior of the star collapses into a black hole. There are also extremely heavy black holes - supermassive black holes, as they are called. They are at the centre of most, if not all, galaxies - and our galaxy is no exception.

At the heart of the Milky Way is something that emits energy in many different wavelengths. It is believed that the energy is coming from gas that is about to fall into the supermassive black hole, Sagittarius A. It has also been calculated that there is something at the heart of the Milky Way that controls the movement of the stars - something that weighs millions of times the weight of the Sun, but occupies an area smaller than the Solar System. The explanation is a supermassive black hole.

Indirect observations
Although we can't see black holes, we have many indirect observations that tell us they exist. We can observe the effect that black holes have on their surroundings - and it's dramatic. If a star or nebula gets too close to a black hole, it is pulled into the black hole's gravitational field.

The dust and gas pulled towards the black hole forms a disc of extremely hot matter that is pulled towards the centre where it eventually disappears into the black hole. Close to the black hole, the gases have a high velocity and emit radiation that we can observe. If the light from the disc around supermassive black holes is very bright, it is categorised as a quasar - one of the most energetic phenomena in the universe.
 

Holes in the universe
We have previously talked about spacetime being the three dimensions that we can see, and also time. Spacetime is a concept Einstein used in his ‘general theory of relativity’ to describe a three-dimensional space with an extra dimension of time. When approaching a black hole, time slows down because spacetime is bent so strongly by the black hole's gravity.

The mysterious centre
There is still a lot we don't know about black holes. Perhaps the most mysterious is the singularity that exists at the heart of a black hole. Here, none of the laws of physics can describe what happens - not even the theory of relativity.

At the centre of a black hole, gravity and density are infinitely large. In theory, an object caught in the gravity of a black hole will be compressed into a single point when it gets too close to a black hole.

The gravity of a black hole is so strong that the speed required to escape exceeds the speed of light. The limit at which light can no longer escape the black hole's gravity is called the event horizon.

If we imagine that an astronaut was inside a black hole and she was trying to send signals out of the black hole, it would not actually be possible for us to pick up the signals - information cannot escape.
 

Future research
There are various experiments and projects trying to describe black holes. The Event Horizon Telescope project is an attempt to observe the event horizon of a supermassive black hole for the first time in history. To do so, scientists have built a virtual telescope as big as the Earth. By joining forces with radio telescopes around the globe, they should be able to create an image of the supermassive black hole at the centre of the Milky Way.

It will be a great moment if we can finally see a black hole directly, especially because there are still many unanswered questions about black holes, such as: What role do black holes play in the evolution of galaxies? What can we say about the relationship between the mass of the black hole and the energy it emits? Can we map the disc of matter surrounding a black hole? How does spacetime behave around a black hole? These are just some of the questions that are being worked on to find answers to in the future.

Observationer

Sorte huller er svære at se, fordi de er meget små, og ja, mørke. Problemet er, at resten af universet også er ret mørt, og det er altså svært at se et sort hul på en sort baggrund. 

Men selv når vi ikke kan se det sorte hul, kan vi ofte observere det indirekte, ved at se på den effekt, som sorte huller har på sine omgivelser – og den er dramatisk. 

Blandt andet kan vi se lyset fra det materiale som det sorte hul er ved at spise. Det kan f.eks. komme fra en stjerne eller stjernetåge, der er kommet for tæt på et sort hul. Så vil stjernen eller tågen blive revet fra hinanden, og støvet og gassen, som trækkes ind mod det sorte hul, former sig som en skive af ekstremt varmt stof, der trækkes ind mod midten, hvor det til sidst forsvinder ind mod det sorte hul. Det meget varme stof vil lyse. 

Man kan også kigge på hvordan det sorte hul påvirker selve rummet omkring sig. Som vi beskrev tidligere, vil det sorte hul jo bøje rummet voldsomt, og når det sker, kan rummet komme til at virke lidt ligesom en linse i en forstørrelsesglas, der vil forstørre og forvride det lys, der ligger bag den. 

En sjælden gang imellem kan man dog være heldig at observere det sorte hul direkte, som det er blevet gjort med Event Horizon Telescope-projektet. Event Horizon Telescope-projektet er første gang i historien, man observerer et supermassivt sort huls begivenhedshorisont (læs mere nedenfor). For at kunne gøre det, har videnskabsmænd bygget et virtuelt teleskop, der er lige så stort som Jorden, ved at forene kræfterne fra radioteleskoper rundt omkring på kloden. Det er ufatteligt imponerende og siden 2019 har man fået to billeder af sorte huller, et af det supermassive sorte hul i midten af M87 galaksen, og et af det supermassive sorte hul i midten af vores egen galakse, Mælkevejen.

Mystikken omkring sorte huller

Vi har tidligere nævnt rumtiden. Rumtiden er de tre dimensioner vi har i rummet (frem og tilbage, fra side til side, og op og ned), og så den fjerde dimension: tiden. Rumtiden er et begreb, som Einstein brugte i forbindelse med sin generelle relativitetsteori for at beskrive et tredimensionelt rum med en ekstra dimension i form af tid. 

Fordi et sort hul bøjer rumtiden, betyder det, at det ikke kun er rummet, der bøjes, men også tiden. Når man nærmer sig et sort hul, går tiden langsommere, fordi rumtiden bøjes så kraftigt af det sorte huls tyngdekraft. Når man kommer helt ind i det sorte hul, vil tiden gå i stå. Ikke for den person der er derinde, men for alle der ser på det sorte hul udefra, Det er meget mærkeligt

Noget af det mest mystiske ved sorte huller er nok singulariteten, som findes i hjertet af et sort hul. Singulariteten er det uendelige lille punkt, der udgør det sorte hul. Her kan ingen af fysikkens love beskrive, hvad der sker – selv ikke relativitetsteorien.

I centrum af et sort hul er tyngdekraften og tætheden uendelig store. I teorien vil et objekt, som bliver indfanget af et sort huls tyngdekraft, blive presset sammen til ét enkelt punkt, når det kommer for tæt på et sort hul.

Tyngdekraften i et sort hul er så stærk, at hastigheden, det kræves for at undslippe, overgår lysets hastighed. Grænsen for, hvor lyset ikke længere kan undslippe det sorte huls tyngdekraft, kaldes begivenhedshorisonten. Når vi kigger på et sort hul, vil der være begivenhedshorisonten vi ser, fordi det er der, det bliver sort. Men det faktiske sorte hul er et lille bitte punkt, der ligger i midten. 

Hvis vi forestiller os, at en astronaut var inde i et sort hul, og hun forsøgte at sende signaler ud af det sorte hul, ville det faktisk ikke være muligt for os at opfange signalerne – information kan nemlig heller ikke undslippe.

Der er stadig meget vi ikke ved omkring sorte huller - både hvordan de opstår, hvor mange og hvor forskellige de er, og hvad der sker inde i dem. Det er et fantastisk mysterium, som bare venter på at blive løst af fremtidens forskere.