Pardon, je to delší, ale odpověď v komentu je také zajímavá.
How Fast Is Dark Matter?
http://aasnova.org/2018/01/02/how-fast-is-dark-matter/
Our galaxy is embedded in a cloud of dark matter, thought to consist of tiny particles traveling along orbits through the halo. These dark matter particles permeate all regions of the galaxy,
extending far beyond the edge of the bright central spiral, but also orbiting through our solar system, and even passing right through the Earth. This is why scientists build giant detectors,
hoping to trap some of these dark matter particles as they pass by. So far, these experiments have not detected dark matter, but that lack of detection is actually quite interesting. Finding
out what dark matter is not, and thereby narrowing down the possibilities, is an important step towards revealing the true nature of these mysterious particles.
In order to really understand what it means when a detecter does not see dark matter, it is important to have a clear prediction for how much dark matter should be detected. For example, if we
expect very few dark matter particles to pass through the Earth in a given amount of time, then maybe the lack of detections over a few years doesn’t actually mean those particles don’t exist.
One essential piece of information in this prediction is the velocity of dark matter particles as they orbit past our solar system.
So, how can we determine the speed of these particles that we haven’t even directly detected? Well, let’s look back at where these particles actually come from. Dark matter halos grow over time
by consuming other dark matter halos. This process is called hierarchical structure formation. The Milky Way is continuously pulling in smaller galaxies and then tearing them apart, thoroughly
mixing their stars and dark matter particles into the Milky Way halo (Figure 1).
This understanding of the origin of these particles reveals an important piece of information: when dark matter particles join the Milky Way, they are often accompanied by stars. This is great
news, because stars, unlike dark matter particles, are not invisible, and we can directly measure their velocities. If we can confirm that dark matter particles tend to move at similar
velocities to their stellar companions, then this problem of determining the local dark matter velocity is much simpler!
Odpověď: Chris Reeves (jde o citaci)
... in some ways, the discoveries made in recent decades have raised as many new questions as they have answered.
One of the most vexing gets at the heart of what our universe is actually made of. Cosmological observations have determined the average density of matter
in our universe to very high precision. But this density turns out to be much greater than can be accounted for with ordinary atoms.
After decades of measurements and debate, we are now confident that the overwhelming majority of our universe's matter -- about 84 percent -- is not made
up of atoms, or of any other known substance. Although we can feel the gravitational pull of this other matter, and clearly tell that it's there, we simply
do not know what it is. This mysterious stuff is invisible, or at least nearly so. For lack of a better name, we call it 'dark matter.' But naming something
is very different from understanding it.
For almost as long as we've known that dark matter exists, physicists and astronomers have been devising ways to try to learn what it's made of. They've built
ultra-sensitive detectors, deployed in deep underground mines, in an effort to measure the gentle impacts of individual dark matter particles colliding with atoms.
They've built exotic telescopes -- sensitive not to optical light but to less familiar gamma rays, cosmic rays and neutrinos -- to search for the high-energy
radiation that is thought to be generated through the interactions of dark matter particles.
And we have searched for signs of dark matter using incredible machines which accelerate beams of particles -- typically protons or electrons -- up to the highest
speeds possible, and then smash them into one another in an effort to convert their energy into matter. The idea is these collisions could create new and exotic
substances, perhaps including the kinds of particles that make up the dark matter of our universe.
As recently as a decade ago, most cosmologists -- including myself -- were reasonably confident that we would soon begin to solve the puzzle of dark matter.
After all, there was an ambitious experimental program on the horizon, which we anticipated would enable us to identify the nature of this substance and to begin
to measure its properties. This program included the world's most powerful particle accelerator -- the Large Hadron Collider – as well as an array of other new
experiments and powerful telescopes.
But things did not play out the way that we expected them to. Although these experiments and observations have been carried out as well as or better than we could
have hoped, the discoveries did not come.
Over the past 15 years, for example, experiments designed to detect individual particles of dark matter have become a million times more sensitive, and yet no signs
of these elusive particles have appeared. And although the Large Hadron Collider has by all technical standards performed beautifully, with the exception of the Higgs
boson, no new particles or other phenomena have been discovered.
The stubborn elusiveness of dark matter has left many scientists both surprised and confused. We had what seemed like very good reasons to expect particles of dark
matter to be discovered by now. And yet the hunt continues, and the mystery deepens.
In many ways, we have only more open questions now than we did a decade or two ago. And at times, it can seem that the more precisely we measure our universe,
the less we understand it. Throughout the second half of the 20th century, theoretical particle physicists were often very successful at predicting the kinds of
particles that would be discovered as accelerators became increasingly powerful. It was a truly impressive run.
But our prescience seems to have come to an end -- the long-predicted particles associated with our favorite and most well-motivated theories have stubbornly refused
to appear. Perhaps the discoveries of such particles are right around the corner, and our confidence will soon be restored. But right now, there seems to be little
support for such optimism.
In response, droves of physicists are going back to their chalkboards, revisiting and revising their assumptions. With bruised egos and a bit more humility, we are
desperately attempting to find a new way to make sense of our world."
That last sentence is really kind of peculiar. Where are these "droves of physicists" who are "going back to their chalkboards"?
I'm having trouble seeing them.