

Why dark matter/energy? - krsunny

Take for example a firecracker, a hand grenade, a star or anything in our universe that explodes. If you were to measure the rate of expansion over time there would be points where the expansion is accelerating, remaining constant or decelerating. At the point in time when the explosion is accelerating, would you call the force driving that acceleration dark energy/matter? No? So why do we invent dark energy/matter to explain the acceleration of our universes expansion?
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btilly
You're asking the wrong question. We didn't invent the concept of dark matter
to explain the acceleration of our universe. Though if we have theories that
have dark matter, we might as well try to make those theories explain the
acceleration at the same time.

Here is why we need dark matter. When something is in orbit, from the details
of their orbit we can tell how much mass they are orbiting around. When we
carry out this calculation for galaxies, we get one mass. We also can estimate
the amount of mass that each visible star has, then add it up to get an
estimate of how much matter there is that we can see. This gives us another
number. The two numbers are off by a factor of 10. The difference is matter
that we can't see, which we know has to be there. This is dark matter.

Our own solar system suggests that the matter in a solar system is almost all
in the visible star. But there are plenty of planet-like things that aren't
around stars. We can't see those, and don't know how many of them there might
be. However our models suggest that those kinds of objects should be
distributed like stars. But we have evidence from the movement of stars around
galaxies that dark matter is relatively uniformly distributed through a bigger
volume than the visible stars.

The most popular (but not only possible!) theory right now is that there are
lots of Weakly Interacting Massive Particles (WIMPs) that only weakly interact
with normal matter. (There is a lot of speculation about the possible nature
of these particles.) Computer models with these result in reasonable
distributions of dark matter. But this theory is difficult to prove or
disprove directly since, by definition, they don't interact with anything we
can build to detect them.

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krsunny
Right after I posted this I realized I only meant to say dark energy, not dark
matter. Why is it speculated that the universe will continue to accelerate its
expansion?

