"It's everywhere, really. It's between the galaxies. It is in this room. We believe that everywhere that you have space, empty space, that you cannot avoid having some of this dark energy." -Adam Riess

Have you heard about the accelerating Universe? What about dark energy, vacuum energy, or a cosmological constant? These are terms we throw around to talk about one of the strangest observations ever made in the Universe: that the more distant a galaxy is from us, the faster it appears to be receding from us. Not only that, but the rate of recession appears to actually increase over time, which is the biggest puzzle of all.

But what does this mean for the expansion rate of the Universe? While you might think, as many do, that this means the expansion rate itself is accelerating, that’s not how it works at all. The expansion rate is dropping today, and will continue to drop, eventually asymptoting to a constant value. So how does this cause acceleration, and what does that mean for the fate of the Universe?

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"There are two things we can measure when it comes to the Universe's expansion: the expansion rate and the speed at which an individual galaxy appears to recede from our perspective. These are related, but they are not the same."

Good stuff! Thank you for the clearly expressed review of the differences.

Hang on! You say at one point that the expansion rate goes down, but near the end you say that current evidence overwhelmingly supports that expansion continues at the same rate forever. (I understand the difference between the expansion rate, also known as the Hubble parameter--formerly Hubble constant--and the apparent recession velocity of a given galaxy, but here you are talking both times about the expansion rate. My understanding was that the Hubble parameter is thought to be decreasing over time--though it is constant across space--and so it's your statement at the end of the article, about the Big Freeze, that is misleading.)

About the "dimension" of the Hubble parameter: it is distance-over-time, divided by distance, which gives 1/time, which is a frequency. Is there any sense to the interpretation of the expansion rate as a frequency?

@Jonathan #2: You need to be a bit careful when you "oversimplify" units like that. At best, it leads to confusion, and at worst it leads to crackpottery :-) Here's a classical example where the confusion is more obvious: Torque is the the application of a force F at some distance R from an axis, and is expressed as the cross product T = F x R. That clearly has units (in SI) of newton-meters, or (kg m/s^2) * m. Work is a form of energy, the application of a force F leading to a displacement d, and is computed as the scalar product W = F.d. As a form of energy, work has units of joules (= kg m^2/s^2), or equivalently kg m/s^2 * m.

"But wait!" you might say. "Those are the same units as before! So is torque really a form of energy? Or is energy really a torque, and the universe is built of little spinning cogs?" That's obviously nonsense: torque is a vector, energy/work is a scalar; it just happens that they carry the same net units.

The Hubble parameter is the same: it "sort of" has a net unit of frequency, but that is just a coincidence, and not meaningful physics. The best way to think about the interpretation of H is as a spatial derivative: H = dv(x)/dx, where v(x) is the actual recession speed we observe of a galaxy at a distance x from us. It turns out that v(x) increases linearly with distance (at least out to a redshift of 1 or 2), so H can be written as a number, with units of velocity/distance. It is much easier to work with H, and to understand its _meaning_, if you keep the units in that form, rather than "simplifying" (and confusing!) to 1/time.

I think if expansion of universe stops someday all matter would join into a single black hole (surrounded by a cloud of dark matter) in the end, because of gravity. But what will happen to all spacetime volume around? Can a black hole suck in spacetime itself? If cannot then how universe could restart?

Also I think if we add together all matter and energy in the universe, the answer is not zero. Also I think expansion of universe cannot go forever because of conservation of energy.