Why aren't there an equal amout of blue and red shifts?

Why are there so few blueshifts in comparison to redshifts? I’d like a real explanation and not: “because more things are moving away from us.”

Thanks for your replies.

Because the universe is expanding. Everything is moving away from everything else.

Because red shift occur both when stars are moving away from us AND when we are moving away from other stars.

We’re not stationary; the solar system is expanding like everything else.

It’s as if we’re in a car. You’re traveling at 50 mph. Another car in front of us in the same lane is traveling at 60 mph. A car in our rear view mirror in the same lane is traveling at 40 mph. The distance between us and the two cars increases at the same rate: 10 mph.

Now with cars we can look at landmarks and check speed; with stars you cannot. But stars traveling faster than use shift away and stars traveling slower than us also show the red shift.

Well someone who really knows astrophysics will be along shortly, but the reason that “more things are moving away from us” is not that we are in a privileged position. The universe is expanding, therefore everything is moving away from everything else.

The standard analogy is to imagine a balloon with spots on it. As you inflate the balloon and the surface gets larger, the spots (representing galaxies) are all getting farther apart.

So anything that happens to be moving toward us and creating blue shift is a local anomaly.

The real reason is that no-one outside the Local Group likes us. :slight_smile:

But my mom says my galaxy is cool…

With the statement in quotation marks you have foreclosed giving you the correct answer.

There are some stars in our galaxy that are moving toward us. I believe the Andromeda galaxy is moving toward us. Some members of the local group might also be approaching. That’s compared to the hundreds of billions of stars in other galaxies and the billions of galaxies all of which are moving away from us.

It’s probably also worth pointing out that most of the red shift of distant galaxies is not actually caused by the Doppler effect, but rather by the expansion of the intervening space during the transit of the light.

IAAAstronomer and this is the key thing. The universe is expanding, and the expansion of the universe is strong enough that everything moves away from everything else, unless things are close enough that gravity can “win” over the expansion of the universe. So everything really is moving away from everything else.

Within our Local Group, gravity is “winning” locally and so from our perspective the galaxies of our local group are blue-shifted as gravity pulls us together. Within other galaxy groups and clusters, gravity also “wins” locally, but with respect to us, the entire group or cluster is moving rapidly away from us, hence we see the galaxies redshifted.

If we were located in one of the galaxies that we see as being redshifted from us, then we would see a very very similar thing to what we observe from our vantage point in the Milky Way – some local galaxies would be blueshifted where gravity is winning but everything else would be redshifted. Thus, our point of view is not “privileged” and anywhere in the universe can view itself as the centre of expansion.

Unfortunately for the OP, the answer “because everything is moving away from everything else, apart from a few local galaxies where gravity is significant” is the correct answer.

RealityChuck, the Solar System is not expanding in the same way that the universe itself is expanding. In fact, because of the scales involved, standard Newtonian Gravity holds throughout the Solar System (apart from some minor relativistic corrections) and the expansion of the universe does nothing to our solar system – Ned Wright’s explanation

Can you point me in the direction of evidence that proves the universe really is expanding.

I think the only evidence is that the red shift of light is real and the only explanation for it that holds up is that the universe is expanding.

A rather famous guy named Hubble figured this one out. He managed to get a telescope named after him.

From here: A Science Odyssey: People and Discoveries: Hubble finds proof that the universe is expanding

I am struggling to understand the difference between something moving away from me and there being more and more intervening space between myself and the thing.

The intervening space is expanding, while the objects in between are inertial, i.e. not under acceleration. This isn’t strictly the case; as part of the local group, we’re being dragged at several hundred km/s by the Great Attractor but other groups and galaxies are still accelerating away from us, and as we get closer to the Great Attractor velocities get higher, so the structures in front of us are accelerating away from us and we’re accelerating from the structures behind us, so nearly everything is redshifted to a degree.

The rate of acceleration to due cosmological inflction is proportional to distance, so objects further away are going much faster. Potentially, there’s no limit to how fast space can expand (spacetime isn’t matter or energy, so SR and GR are mum on its rate of propagation) resulting in an event horizon where the objects are moving faster than c (the speed of light) away from each other, and thus can never communicate.

Stranger

Somewhere I saw a reproduction of Hubble’s plot of red shift vs. distance. I’m here to tell the world that it took guts to stand up before a group of scientists and say that the plot showed a linear relationship between red shift and distance.

Well, there’s the fact that there are so few blueshifts in comparison to redshifts. Cite: Your original post.

Wikipedia provides a copy of Hubble’s original Powerpoint slide: Hubble’s Law.

With regard to the Great Attractor, APOD has a lovely image of the dipole in redshift produced by our movement towards it: CMBR Dipole: Speeding Through the Universe Lest anyone read the caption too closely, and become confused by the rough equality between red and blue shifting; the colors are normalized to show small variations in what is a large, remarkably uniform red shift (details here).

Well, the reds have such a short life expectancy, so we need a lot more of them. And the blues are major characters or guest actors.

… what?

:smiley: :smiley: :smiley:

Bravo, sir.

(bolding mine). Powerpoint? In the 1920s? Wow, Hubble really was way ahead of his time.

:wink: