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Meaning, how would we see the universe if the Hubble constant was not 72.5 (km/s)/Mpc, but -72.5 (km/s)/Mpc?
One consequence we should think about is for how we figure out what the age of the universe is. If the universe started out as a singularity state, and has expanded at a constant rate to the present time, then it is simple to find the age by taking the inverse of the Hubble constant:
Distance = (velocity)x(time), so time = distance/velocity. But the expansion rate (Hubble constant) has dimensions of velocity/distance, so 1/(Hubble's Constant) has dimensions of time and equals the age. The way to think about this is to imagine a stretchy rubber cord, with a bunch of evenly-spaced dots along it. If you stretch the cord at a constant rate, measure the speed at which two dots move away from each other, and divide by their current separation, then this is the "Hubble constant" for that cord, and the inverse of that tells you how long ago all the dots were on top of each other -- assuming that the expansion actually extrapolates back that far.If H = 70km/s/Mpc, then 1/(70km/s/Mpc) works out to be 4.4x1017 seconds, or about 14 billion years. (The real expansion rate actually varies with time, depending on the density of matter, radiation, and dark energy, so this turns the calculation of the age into an integral, but it is still the same idea, and in fact this is exactly how we determine the age of the real universe).
If the universe is instead contracting, then this logic doesn't work. What was the initial size of the universe? Has it been contracting forever? Instead of using the inverse of Hubble's constant, we would need to estimate by other methods, like the universe must be at least as old as the oldest objects we can find and measure ages of. We could also use light-travel-time arguments with the most distant objects we can observe, since the universe must be at least as old as the time it takes for light from those sources to reach us.We could make things simple by considering a contracting universe that is the same age as the real universe but contracting at ~70km/s/Mpc. What will this look like?
- Instead of distant objects appearing redshifted, they will look blueshifted (just as you imagined). Photons will be "squeezed" by the contraction of space, shortening their wavelength and increasing their energy.
- The distant (older) universe will be seen to have a lower density than the nearby (younger) universe. The density increases with time. This also means galactic collisions will become more frequent, until inevitably there is a "Big Crunch" (unless something halts the contraction).
- Very distant galaxies may approach us faster than light. This does not mean that they move through space faster than light (they may be moving very slowly relative to the space in their neighborhood), but rather that the space between them and us is contracting fast enough so that Hubble's Law will yield |v|>c (negative v in this case), just as there are objects in the real expanding universe that have recession velocities greater than c.
Aside: One important, counter-intuitive, and very
frequently misunderstood fact that I'd like to mention here: not only are there distant galaxies in the real universe that recede from us faster than light -- we can also see them. There are even galaxies that we observe which are not only receding faster than light now, but have always had recession velocities faster than light.
It is difficult to visualize how this is possible, but it can be made sense of by thinking about how the photon moves through the expanding space, at the same time that the expansion rate changes. (Before dark energy took over, the expansion rate was decreasing, so the distance at which the recession speed exceeds c was increasing). So some photons cross over that boundary and were then able to reach us, even as the sources were never inside that region. There is a wonderful paper "Expanding Confusion" that explains this and many other common misconceptions about the superluminal expansion in great detail (highly technical but worth looking at).
An'shur wrote:You are right that ultraviolet and shorter wavelengths can be shifted into the visible range by the expansion (in fact this sometimes caused confusion when looking at highly redshifted spectra -- we might have a difficult time identifying spectral lines in the optical, because they are actually ultraviolet lines!) However, this doesn't necessarily keep the redshifted spectrum as bright because there is usually less light emitted overall in those wavelengths. The majority of light from galaxies is emitted by the stars, and the majority of that is in infrared to visible (depending on the typical ages of the stars -- spirals tend to be bluer than ellipticals for example). So there is typically less UV and higher energy light being shifted into the visible to replace what was shifted out of it. The other problem is that the redshift reduces the total brightness (over all wavelengths) that we receive. Think of it like this: not only is each photon being stretched out, but the photons are also being pulled apart from one another. So we receive fewer of them per unit time, regardless of what their wavelength was. The same thing will happen to the image of something falling into a black hole -- it not only turns redder, but also fainter, and eventually vanishes in all wavelengths, even though we as outside observers say it never crossed the horizon.
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Which gets me to another question. Why are the galaxies in the deep field images so red? Even if the whole visible spectrum was redshifted far into infrared, shouldn't ultraviolet or shorter wavelengths shift into the visible spectrum, making the galaxies visible?