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Black holes are invisible.
So does the mysterious dark matter in the universe.
To date, scientists have not been able to answer the question of what dark matter is made of.
But there may be a connection between black holes and dark matter.
The physicist and astronaut Ulrich Walter speaks about this in this interview and on “Spacetime” on WELT television.
WORLD:
In the center of our Milky Way there is a huge black hole - also called Sagittarius A *.
Its mass corresponds to that of 4.3 million suns.
Some people fear that our earth could be swallowed up by this monster at some point.
Is this concern justified?
Ulrich Walter:
No, not at all.
We are far too far from this black hole for that.
Our solar system, and thus also the earth, orbits the center of the Milky Way at a distance of around 30,000 light years - and has been for 13 billion years.
This will not change in the future either.
“Spacetime” with Ulrich Walter on WELT
The fourth season of the documentary series "Spacetime" with Ulrich Walter starts on Sunday, February 21 at 6:05 pm on WELT.
Source: WORLD
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WORLD:
But the black hole could emit dangerous radiation if it swallowed larger amounts of matter than usual.
Is that a risk for us?
Walter:
It is true that black holes emit intense X-rays when they devour matter.
If we were closer to the center of the Milky Way, this radiation could actually be dangerous to us.
But we are far enough away.
We live in the habitable zone of our galaxy.
WORLD:
Galaxies also have a habitable zone?
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Walter:
Absolutely.
Usually one hears the term habitable, i.e. habitable zone, in the context of exoplanets.
So that there can be life on a planet, it must not be too close and not too far away from its central star.
Otherwise it will be either too hot or too cold on this planet.
The distance range in which life is not fundamentally excluded is called the habitable zone.
It is similar with galaxies.
If we were too close to the center of the Milky Way, we would be affected by the high-energy radiation from the black hole.
In addition, the density of stars in the inner region of the galaxy is greater and thus the probability that there will be a supernova explosion in the cosmic neighborhood.
The radiation released during such an event can also wipe out life on a planet.
At the outer edge of a galaxy, on the other hand, there is not yet enough solid matter from which planets can form.
All heavier chemical elements must first of all arise in a supernova explosion.
This, in turn, assumes that there must be a sufficient number of old stars.
Our solar system and thus our earth is in any case in the habitable zone of the Milky Way, in which all the conditions for the emergence of life on planets can be met.
WORLD:
Over which area does the habitable zone in our Milky Way extend?
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Walter:
Measured approximately in a range of 10,000 to 40,000 light years from the center.
WORLD:
Do all galaxies have a black hole in the center?
Walter:
Most likely yes.
All stars heavier than three solar masses end up as a black hole.
Since the star density in the center of a galaxy is very high, a particularly large number of these stellar black holes form there over time.
These in turn gradually eat each other up and grow into what is known as a supermassive black hole.
Sagittarius A * in the center of our Milky Way must also have grown in this way.
Because a black hole with 4.3 million solar masses cannot logically have arisen directly from just one star.
Most astrophysicists today assume that all galaxies actually have at least one supermassive black hole in the center.
WORLD:
So there are surely still many stellar black holes in our Milky Way that will sooner or later merge with Sagittarius A *?
Walter:
That's exactly how it is.
And we're not talking about a couple of copies.
The number of stellar black holes in our Milky Way is estimated to be around 100 million.
Like Sagittarius A *, we cannot see them.
There are essentially only two ways to prove the existence of a black hole.
Either you measure the orbit of a star that orbits a black hole at a relatively short distance.
In this way, Nobel laureate in physics Reinhard Genzel was able to prove the existence of Sagittarius A * and at the same time determine its mass.
The other possibility is to prove the radiation that black holes release when matter is swallowed.
Neither method will work on stellar black holes.
Due to the large distances to other objects, matter very rarely flows into them.
And only with larger amounts of matter would there be a chance at all of detecting the resulting radiation with sensitive telescopes.
This is another reason why the measurement of gravitational waves is so interesting.
When two black holes merge, they emit gravitational waves.
This is then also an indirect proof of the existence of these black holes.
This measuring method has only been available to astrophysicists for a few years.
During this time, so many black hole fusions were observed that such processes are apparently more frequent than previously thought.
However, none of these events took place in our Milky Way.
WORLD:
Even if there is a massive black hole in the center of our Milky Way, its gravity plus that of all visible stars is not enough to prevent our rotating galaxy from flying apart.
But obviously our galaxy is quite stable.
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Walter:
That's right.
The mass and thus the gravitational force of the stars is, in view of the large distances between them, not sufficient to keep the spiral galaxy stable in its known form.
In the inner part of the galaxy there is enough matter to hold the Milky Way together.
But the edge of the galaxy rotates at such a high speed that it should actually fly away.
Astrophysicists have calculated that there would have to be five times more matter in order for the Milky Way to be stable.
And so the hypothesis arose that there must also be dark matter, i.e. matter invisible to us, which holds the galaxy together with its gravity.
And there is another indication of the presence of dark matter.
The orbital speed in our galaxy is independent of the distance to the center.
Actually, according to the laws of classical physics, it should decrease with increasing distance.
However, additional dark matter in the outer area of the galaxy could explain this anomaly in the rotational speed.
WORLD:
But so far the astrophysicists have not yet found out what this dark matter actually consists of.
How is the state of affairs there?
Walter:
At first it was thought that dark matter is a new type of elementary particle that only interacts with the environment via two natural forces - via gravity and via what is known as weak interaction.
These hypothetical particles have been called WIMP.
This acronym stands for “Weakly interacting massive particle”.
Because dark matter does not interact electromagnetically - especially not with light - this means that it is invisible.
It also has the consequence that you simply cannot touch this matter.
My hand would just reach through a piece of dark matter without my feeling anything.
On the other hand, WIMPs still have to be very massive in terms of a strong gravitational force.
Therefore the neutrino is ruled out as a candidate for dark matter.
Neutrinos are electrically neutral and are only subject to weak force and gravity.
But they are just not massive enough.
The neutrino mass is far too small to explain the aforementioned effects in the Milky Way.
In addition, neutrinos fly through space at almost the speed of light.
Therefore they cannot be a stationary gravitational cement in a galaxy.
WORLD:
If there are no neutrinos, what other particles are possible?
Walter:
Researchers have targeted two hypothetical particles in particular - neutralinos and axions.
They were searched for intensively with specially built detectors.
So far in vain.
The theorists then came up with a so-called MOON theory.
MOON stands for "Modified Newton Dynamics" and says that the effect of Isaac Newton's good old gravitation theory is modified at extremely large distances in such a way that the stability of the outer parts of galaxies can be explained on paper.
This is of course a fundamentally different approach.
In this case one no longer needs to look for any dark matter, but the matter known to us then simply interacts with a correspondingly changed gravitational force.
WORLD:
Are you a supporter of the MOON theory?
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Walter:
I personally prefer another option.
Quite a few researchers now consider it plausible that the dark matter in our Milky Way consists of much more than these 100 million "small" black holes.
Accordingly, the Big Bang created so-called primordial black holes, which typically only have as much mass as our earth.
Such comparatively small black holes can only arise at extremely high densities and energies, as was the case with the Big Bang.
WORLD:
Before the world's largest particle accelerator went into operation at the Geneva research center, Cern was warned at the time that when particles collide at very high energies, black holes could form, which could then swallow the earth.
Walter:
Exactly.
When particles collide in the accelerator with such high energies, extremely tiny black holes can indeed be created.
But they are completely harmless.
They dissipate within a very short time and then disappear again.
After all, their mass is tiny, only as large as that of two atomic nuclei.
These black holes have no chance of growing further and gradually swallowing up the whole earth.
Earth-mass black holes, however, can exist for billions of years.
The theory predicts that they will remain stable for around 1050 years.
So all primordial black holes the size of a mountain or larger, if they existed, would still exist today.
And so in our Milky Way there is not only the supermassive hole Sagittarius A * and millions of stellar black holes, but possibly billions or even billions by billions of such mini-holes.
They could be spread all over the Milky Way.
Nobody knows how much it really is.
Their masses could range from that of a stone to that of a large planet.
WORLD:
How many followers do these different theories on dark matter have?
Walter:
Until recently, around 80 percent of physicists believed in the Neutralino.
But that has changed in the last two or three years.
With the many search experiments that have been carried out over the years, the Neutralino should have been discovered long ago - if it existed.
This has led to a certain disillusionment.
At the moment the experimenters are concentrating on the search for axions.
I estimate their following at 40 percent.
Around 20 percent rely on the MOON theory, and the primordial black holes favor 20 to 30 percent.
So there is currently no majority opinion on this issue.
WORLD:
How could one prove the existence of primordial black holes?
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Walter:
Even if the distances between the primordial black holes are large and the interstellar space contains very little matter, from time to time some matter should fall into such a black hole and a small lightning bolt would arise.
In fact, there is a research project that looks for such flashes on the edge of our solar system.
The hypothesis of a so-called planet X was derived from the orbital data of the outermost minor planets in our solar system, the so-called trans-Neptunian objects.
It is possible that this planet, which has not yet been detected by telescopes, is a primordial black hole.
"Spacetime" Season 4 - Episode Overview:
The first of the six new episodes of “Spacetime” will be broadcast on February 21 at 6:05 pm on WELT.
The others then follow at the same time on the following Sundays.
The dark side - black holes and invisible matter
Gravitation - the force that holds everything together
Astrobiology - The Search for Life in Space
Silent companions - moons in our planetary system
Space Junk - Fast and Dangerous
Destination Mars - From the Moon to the Red Planet