The accelerating expanding universe

I am finding it difficult to comprehend the concept that the expansion of the universe is accelerating. If the expansion is accelerating today, then presumably this acceleration has been taking place for billions of years.If the divergance of a given galaxy is increasing at a rate of only 1 mile per hour/year, then that galaxy would be travelling away from us at the speed of light within only 700 million years. Clearly this is not happening, so the rate acceleration must be very small indeed. Is it really possible to measure such a microscopic rate of acceleration by observing such a distant object?If any acceleration is taking place, I would expect every other galaxy to be so far away by now that we could not possibly see them. Or....If the llight could reach us, those other galaxies would be diverging at such a colossal speed that I would have thought that the light reaching us from them would no longer be within the visible spectrum.Edited by Hoof Hearted, : No reason given.

Thread copied here from the The accelerating expanding universe thread in the Proposed New Topics forum.

I think your first mistake is assuming we are the center of the universe. That skews your whole outlook on the subject.

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"Some people think God is an outsized, light-skinned male with a long white beard, sitting on a throne somewhere up there in the sky, busily tallying the fall of every sparrow. Othersfor example Baruch Spinoza and Albert Einsteinconsidered God to be essentially the sum total of the physical laws which describe the universe. I do not know of any compelling evidence for anthropomorphic patriarchs controlling human destiny from some hidden celestial vantage point, but it would be madness to deny the existence of physical laws."-Carl Sagan"Show me where Christ said "Love thy fellow man, except for the gay ones." Gay people, too, are made in my God's image. I would never worship a homophobic God." -Desmond Tutu

I wasn't making that assumption at all. Apart for a few exceptions, the distance between every galaxy and it's neighbors is expanding and accelerating. So the same observation could be made from anywhere.

You are on the right track actually, let me make a few corrections off the top of my head:You are right in saying that the acceleration has been taking place for billions of years, but your rate of increase is probably off. The interesting thing is that the more distant a galaxy, not only the faster it appears to be receding, but also it appears to be increasing in that speed faster than closer galaxies. In other words, the speed at which a galaxy recedes from us appears to be proportional to the amount of space between us and the galaxy.My point is that a constant rate of increase is not fully accurate, so if we observe 1 mph/y now, that wasn't always the case. Also, it is certainly possible to measure small rates of acceleration of such distant objects.

One thing to consider is that the galaxy itself is not moving, it is the space between the galaxies that is expanding.The further away a galaxy is from where ever you measure it from, the faster it will appear to be accelerating because spacetime is highly curved at cosmological scales. In an observational sense, it could appear to be exceeding the speed of light, meaning that one galaxy can't be observed from the other galaxy. But that doesn't mean it's violating the speed of light in a physical sense.- Oni

Good reply (as is Oni's) - my only concern is this bit: No, unfortunately it isn't. Clearly it is not possible to measure the acceleration in real-time given the time-scales this acceleration is acting over. And the local motion of any particular object (galaxy, quasar, cluster) will totally drown out the acceleration. What you need to do is look at large groups of objects of a range of red-shifts and infer the acceleration statistically. Even now I'm not sure of the range of possible rates of increase of expansion, and the most sensible statement is still probably that:the null hypothesis of "no rate of increase of expansion" can be discarded in favour of the alternative hypothesis of "there is a positive rate of increase of expansion" at a very high degree of confidence.Edited by cavediver, : No reason given.

Please tell me if I have this right. Would this mean that Hubble's constant is not constant? That is, is Hubble's Constant higher for more distant galaxies than it is for closer galaxies (assuming they are not gravitationally bound)?

This stuff is way outside my area of expertise (or even minimal education), but I have a question. The further away an object is, the older we are seeing it, simply because it takes the light longer to get here. Thus, isn't it reasonable to assume that we are seeing past acceleration? If this is the case, what reason is there to conclude that the past acceleration is still occurring?

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Ridicule is the only weapon which can be used against unintelligible propositions. Ideas must be distinct before reason can act upon them; and no man ever had a distinct idea of the trinity. It is the mere Abracadabra of the mountebanks calling themselves the priests of Jesus. -- Thomas JeffersonFor we know that our patchwork heritage is a strength, not a weakness. We are a nation of Christians and Muslims, Jews and Hindus -- and non-believers. -- Barack ObamaWe see monsters where science shows us windmills. -- Phat

Not much Except that our coarse estimates of the acceleration show it to be fairly constant over the periods examined. If dark energy really is a result of a true Cosmological Constant, then it is constant. If it is the result of a dynamical field (quintessence), then it could well vary with time and may decrease, or even increase (remember the Big-Rip?) But observation suggests that any change is slow on the scale of the few billion years covering the earliest observations to present day.

Cavediver can correct this if I'm wrong but, Hubble's constant is exactly how Phage described it: it's directly proportional to the distance.See: source

quote:

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The law is often expressed by the equation v = H(o)D, with H(o) the constant of proportionality (the Hubble constant) between the distance D to a galaxy and its velocity v. The SI unit of H(o) is s-1 but it is most frequently quoted in (km/s)/Mpc, thus giving the speed in km/s of a galaxy one Megaparsec away.

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- Oni

Err, no.Hubble's law is the observation that galaxies are receding from us at a speed proportional to their distance from us. Hubble's constant is the constant of that proportionality, usually stated in (km/s)/Mpc which is kilometers per second for a galaxy one Megaparsec away.So, what this means is that a galaxy one Megaparsec away is receding at a particular speed, and one two Megaparsecs away would be receding at twice that speed. Since the more distant galaxies have a higher velocity they are increasing in distance faster than slower, closer galaxies; ergo, more distant galaxies appear to accelerate away faster than closer galaxies.If they go faster with more space between us, getting more space between us faster means they go faster... faster. Even though Hubble's constant is constant.

If this helps, currently I believe the estimated value of the constant is: 71 (+/- 7) km/s/Mpc. Which is roughly about 13-14 miles per second per million light-years.- Oni

I have a couple questions maybe Cavediver can answer.1. If the observations of accelerating expansion are correct is this because the expansion has moved galaxies or clusters of galaxies far enough apart that their mutual gravitational attraction is counteracting some expansion force of the Universe to a lesser and lesser degree?2. Was the inflation period/event the Big Bang or something that happened after the Bang? And is the expansion we observe today an after-effect of inflation, kind of like "coasting"?thanks

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What if Eleanor Roosevelt had wings? -- Monty PythonYou can't build a Time Machine without Weird Optics -- S. Valley

Sure:1) No - this is a bit mixed up, and rather Newtonian.If we stick with the Newtonian picture, we can describe the old non-accelerating Big-Bang cosmology: the galaxies are flying away from each other following some initial "push". Their mutual gravitational attraction is slowing them down, but how much slowing is a function of how many galaxies there are per volume.There are two principle scenarios: a) the galaxies are sufficiently far removed from each other and moving suifficiently quickly that although they will always continue to slow, they will never stop moving away. This is the Open Universe; and b) the galaxies will eventually slow, halt, and start to move towards each other under their mutual gravitational attraction. This is the Closed Universe. There is a c), the Flat Universe, which is the limiting case between a) and b), where the galaxies will continue to expand, but "only just".Now we add to this the "dark energy", the on-going "push" that is causing the galaxies to move apart ever more rapidly. This is sufficient to ensure that the galaxies will never halt. But the on-going push is an active force independent of both the original expansion push of the Big Bang, and the gravitational attraction of the galaxies.2) Inflation occured very soon after the Big Bang, within the first second of the Universe. The expansion of the Universe is unrelated to inflation - inflation was an extra boost on that inflation - but it is possible that Inflation and the acceleration of the expansion are related, as these are very similar in nature. We call the field responsible for driving Inflation the Inflaton field, and the field for driving the acceleration is Dark Energy. It is just about possible that these could be the same field, behaving with very different energy scales at different times in the Universe.