This goes back to the “spots on the surface of a balloon” analogy – two spots painted on an expanding balloon will be further and further from one another as the balloon expands, although neither spot is in motion relative to the fabric of the balloon; the growing distance is due to the fabric of the balloon itself stretching, not to the spots moving.
The magnitude of the Doppler redshift is determined by the relative velocities of the emitter and the receiver, regardless of the distance between them. The magnitude of the cosmological redshift is determined by the distance between the emitter and the receiver, regardless of their relative velocity. For nearby objects the former mechanism dominates, and for distant objects the latter does. The dominance of redshifted objects in the night sky is primary a result of the cosmological redshift and not the Doppler redshift.
The two types of redshift produce different observational effects, particularly when the light interacts with an intervening object.
One way to think of it is that when light travels a really long distance it “decays” along the way as the vacuum expands around it. The popular conception of the Big Bang is that the galaxies are speeding apart as the result of a huge explosion. But that’s not really what’s going on. Rather, the fabric of space itself is relaxing, so things get farther and farther apart without actually moving.
The linear increase in redshift with distance, and the Hubble constant’s independence from which part of the sky you are looking at puts the lie to the big explosion scenario. Unless you assume that the Milky way is at the exact center of the universe, an explosion would result in a redshift/distance relationship that varied depending on which direction you look relative to the center of the explosion.
However, Hubble’s data is consistent with a universe in which expansion takes place uniformly, throughout space; rather like a rising loaf of bread with embedded poppy seeds for galaxies.
Well, the Milky Way is the Center of the Universe… and so is every other point in the universe.
If you take a theoretical point-sized balloon and start blowing it up, there are no privileged points on the balloons surface – all points on it are equally the “original” point, in a sense.
You know I thought that one of the main points of relativity was that there is no surface of the balloon to measure your position relative to. Even if you take the balloon analogy the spots are moving.
The last eighty years of astronomy may be of interest to you.
Like all analogies, this balloon comparison breaks down at some point, but with a little mental flexibility it illustrates the concept of inflation very well. The surface of the balloon, which is two dimensional (i.e. if you’re a dot on the balloon you can’t go up or down, just forward back, left and right) is analogous to the three dimensional space we move about it. Given a perfectly spherical balloon (and using a spherical coordinate system for symmetry) the dots are only moving radially outward. In fourspace, this would effectively be the time dimension, in which everything is constantly in motion (except for photons, which follow a geodesic that is invariant in time; that is, a photon has no sense of time “progressing”).
Inflation is the simplest explanation for a number of different phenomena in cosmology, and has the virtue of fitting our observations without too much mental gymnastics. However, we can’t actually observe space itself to expand, only the effects on other matter and energy, hence the redshift. It’s entirely possible that the idea is incomplete or absolutely wrong, but as yet no one has proposed a suitable alternative that satisfies the questions that cosmologists and astrophysicists have to the same extent.
Stranger
I am still not seeing the distinction between space expanding and things moving. Perhaps the question to ask is what observations could you make to tell the difference between the two.
Is there really something other than the red shifts that indicates the universe is expanding? I’m asking because I don’t know, not because I doubt it. I think that cephid stars in distand galaxies would be just too far to get accurate measurments, and I don’t know what other factors can be used to measure distances.
What if photons gradually irradiated energy as they traveled, and this resulted in red-shifts?
As I am a chemist please describe these astronomical phenomenon in terms of quantum mechanics rather than general relativity. (Err joke)
Consider a universe containing two objects (galaxies, say), which can interact via light. The universe is initially static, so the two galaxies are staying at a constant distance from each other. Galaxy A emits a pulse of light, which travels to Galaxy B. While the light is en route, something happens, and the universe expands, doubling in size. Then, the expansion halts while the light is still en route, and when the pulse is received, the universe is again static. In this hypothetical universe, there would be no Doppler shift due to motion, but there would be a cosmological redshift due to the expansion, and the light received would have twice the wavelength and half the frequency of the original light.
Christopher, Cepheids are indeed only useful for relatively close galaxies, but for more distant galaxies, there are other standard candles which can be used. The brightest are the type 1a supernovae, which can be observed nearly out to the edge of the observable Universe. The technique is basically the same as with the Cepheids, just with a brighter source: We know or can determine how bright the source inherently is, and we see how bright it appears to be in the sky, so we can determine the distance.
If I understand correctly what you’re trying to get at here, then this argument doesn’t actually work.
Imagine an explosion in a classical universe. As the debris spreads from the origin, the change in its distribution with time is just equivalent to a dilation. But it’s simple geometry that a dilation preserves shape, e.g. a triangle of three points remains the same shape, but is expanded by the scale factor, even if none of the points is at the origin. From the point of view of an observer on one of the vertices, this looks just like the naive classical Doppler Shift interpretation of Hubble’s Law. The other two points are moving away with a speed proportional to their distance. Otherwise the shape of the triangle isn’t preserved.
But this applies to any observer on any piece of the debris looking at any other pieces. They see a linear velocity-distance relation for everything. In this respect at least, what they see doesn’t depend on what direction they look in even though the explosion had a definite centre.
Of course, this isn’t what is happening in the real universe. But there are other reasons for believing that.
One of the early objections to the red shift resulting from expansion was that the light lost energy (tired light) over time. However, as far as I know, no instance has ever been discovered of a photon radiating away a part of its energy as another photon nor have any mechanisms been suggested for such an event to happen.
Pick a point somewhere within the expanding mass. Plot redshift velocity vs distance along the direction of expansion, e.g. away from the center. Compare that to a plot of velocity vs distance along a line tangent to the expansion front. Along that line, you won’t see as strong a dependence of redshift vs distance because most of the velocity is in a direction that doesn’t contribute to the observed redshift.
I don’t think you’ve grasped the counter example at all. The right-angled triangle formed by the observer, the object at a tangent and the object on a radius merely expands as the scale factor of the dilation. The observed component of velocity away from the observer for both objects is proportional to their distance (with the same factor).
The point to the original question by gazpacho is: how would this be observably different than if the two galaxies were actually moving apart in space at the same rate (that is, sufficient to cause a redshift doubling the wavelength)?
The answer to the question as I understand it is that there wouldn’t be any difference, but unless we have the unique quality of being at the dead center of the observable universe, if the cause of the redshift we observe in all directions was actual motion of the galaxies within space, then we would have to assume that essentially all objects are moving away from us in all directions, which would presumably only be possible at the “center” of the observed universe. Ergo, we take the much more plausible assumption that it isn’t the motion, but the expansion, that creates the observed shift.
I think I see the trouble: “As the debris spreads from the origin, the change in its distribution with time is just equivalent to a dilation.” While it’s possible the expansion resembles a photographic enlargement, that requires that objects increase their radial velocity as they move out from the center; increase their velocity after the initial impetus of the explosion. There’d need to be a force to accomplish that acceleration, but such a force is lacking in what we usually call an explosion. In an explosion, some objects come out fast, some slow, but once the explosion is done all the objects continue to move at the same speed, unless acted on by an external force.
The explosion I’m visualizing isn’t just a dilation.
There’s no external force after the explosion. Stuff that’s fast at the outset stays fast at exactly the same speed. Stuff that’s slow likewise stays slow. Every object obeys the relation r = vt for its distance from the centre. The radial velocity v is constant for each object, but it can be different for different objects.
But t then just acts as the scale factor. Such an explosion does just look like a dilation.
Even this argument doesn’t wash; we’re clearly not in the dead center of the universe, as we are in motion (at at 300 to 600 km/s) against the cosmic background toward the Great Attractor. Either the Milky Way is being held fixed while other structure is being made to flow around it in the simulacrum of motion, or we’re just one insignificant raisin in the dough. Inflation doesn’t just explain the cosmic redshift, though; it also satisfies a whole bunch of other problems in cosmology, including the almost completely homogenous distribution of matter in the observable universe, the apparent near flatness of space, thermodynamic equilibrium in SR across distances which are causally unconnected, et cetera.
Stranger
Guys, there is no explosion. Despite the moniker (deridingly applied by Fred Hoyle) to the genesis event as the Big Bang, there was no “bang”; just a rapid expansion of space. The first “bang” occured somewhere around 300-400k years when space expanded enough that ordinary matter could condense and photons could move freely as the universe became transparent. The cosmic microwave background is the residue of that event. Consider it the universe’s first word.
Strange
We know, but if there had been, it’d have to have some pretty odd properties to match what we see today.