The Hubble-constant gives the rate at which the universe expands at the present. Since the expansion of the universe hasn't been constant this is indeed not the case.
So yes, the Hubble-constant is actually a time-depandant value the name constant dates back to the time people believed the universe was static. The behaviour in time of the hubble constant H yields whether the expansion speeds up of slows down. One can make models for the universe. The simpelest model is the Robertson-Walker universe in which one can derive the Friedmann-equations.
These Friedmann equations give the time-evolution of the Hubble-constant in this universe. I don't know what the actual situation is of the cosmological models, but I believe it's the same method of working. Sign up to join this community. The best answers are voted up and rise to the top.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. The first Friedmann equation is what you arrive at if you start with a Universe that's uniformly filled with matter, radiation, and whatever other forms of energy you want.
The only assumptions are that the Universe is isotropic the same in all directions , homogeneous with the same average density everywhere , and governed by General Relativity. If you assume this, you get a relation between H , the Hubble rate on the left-hand side , and all the various forms of matter and energy in the Universe on the right-hand side. The first Friedmann equation, as conventionally written today in modern notation , where the left Interestingly, as your Universe expands, the density of matter, radiation, and energy are allowed to change.
For example, as your Universe expands, its volume increases, but the total number of particles within your Universe stays the same.
This means that, in an expanding Universe, for:. As time goes on, a grows, and therefore different components of the Universe become more-or-less important relative to one another.
How matter top , radiation middle , and a cosmological constant bottom all evolve with time in A Universe with a greater overall energy density has a greater expansion rate. On the contrary, one with a smaller energy density has a lower expansion rate. As the Universe ages, it expands; as it expands, the matter and radiation within it becomes less dense; as it becomes less dense, the expansion rate drops.
The expansion rate, at any given time, determines the value of the Hubble constant. In the distant past, the expansion rate was much larger, while today it's the smallest it's ever been. Various components of and contributors to the Universe's energy density, and when they might If cosmic strings or domain walls existed in any appreciable amount, they'd contribute significantly to the expansion of the Universe. There could even be additional components that we no longer see, or that haven't appeared yet!
Note that by time we reach today, dark energy dominates, matter is still somewhat important, but radiation is negligible. So why, then, you might wonder, do the very distant galaxies we observe appear to follow this straight-line relation? It's because all of the light that arrives at our eyes, from the light that was emitted by a galaxy next door to the light that was emitted from a galaxy billions of light years away, is all The age of everything in the Universe, by time it reaches us today, has lived through the same ever-changing Universe that we have.
The Hubble constant was higher in the distant past, when much of the light was emitted, but it's taken billions of years for that light to arrive at our eyes. Light may be emitted at a particular wavelength, but the expansion of the Universe will stretch it The researchers were able to extrapolate their evolving Hubble constant back to the time of the cosmic microwave background and match it up with those results.
The new results are not altogether surprising. In this case, the researchers added a new variable — how quickly the Hubble constant changes with time — and they were able to find a way to connect the early- and late-time measurements of the Hubble constant. Also, the work did not find a statistically significant measurement of this varying Hubble constant. Although they were able to relieve the tension in cosmological observations, they were not able to conclusively say that the Hubble constant is changing with time.
These results, if they hold up, could give theorists a pathway to introducing new physics into the universe to explain the Hubble constant tension. Because of the independent corroborations, Riess has become more confident there must be a fundamental discrepancy involved, one that is not due to methodological flaws or mistakes in observations but caused by a feature of our universe of which scientists have had no previous inkling.
But if measurement error can no longer be considered a cause of the differences in Hubble constant values, what new concepts could explain this discrepancy? Astronomers have already put forward a number of suggestions.
One idea proposes that the universe contains a new class of subatomic particle that travels close to the speed of light. These entities are called dark radiation and could also include already known particles such as neutrinos.
Another idea is there was a special, intense dark-energy episode not long after the big bang, which expanded the universe faster than astronomers had previously appreciated.
And finally there is the possibility that the particles that make up dark matter interact more strongly with normal matter than previously assumed. Again this would have an impact on the Hubble constant. Not every scientist is over the moon about the prospect that one of these proposals is the answer to their measurement quandary and still hope that in the end it may be possible to reconcile the two values they get for the Hubble constant.
This point is stressed by Mortlock. They are two huge question marks that are already hanging over our understanding of the cosmos. Personally, I do not feel the need for a third. For his part, Riess takes a slightly more optimistic view. That suggests we are heading in the right direction in understanding the universe — though it just may be that we have at least one other step to take. Stars known as Cepheid variables have played a critical role in our understanding of the expansion of the universe.
These stars, which are relatively common, vary in brightness over periods of days or weeks.
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