Redshift*speed of light = Hubble Expansion + peculiar motion If you're a fan of equations, then conceptually, redshift works like this: That's really the key to understanding cosmology, is to recognize the best way to put all the puzzle pieces together. But if all you measured was the redshift of one object, there would be no way to discern. Mu, physically, "movement" leaves other signatures besides redshift. My question is, would the photon still have travelled n wavelengths? In other words, if n1=x/lambda in the first scenario (where lambda = the photon's wavelength = constant), would n2=int(dx/lambda) = n1 in the second scenario (where lambda = f(x))? It would also travel more distance and take more time to reach it's destination than the first photon. If we repeat this process but this time with expansion, then your number 3 says that the expansion of space would stretch the photons wavelegth throughout it's journey until it hit the second object with some wavelength greater than it started with. The distance between the objects is n times the photon's wavelength, and there is no expansion of space between the objects, so the photon travels n times it's wavelength before hitting the second object. Let's say that two objects are some distance, x, apart, and a photon leaves one and heads for the other. Hope this helps shed some light on some of the most confusing stuff out there! Again, it's expansion that's causing this redshift, and not motion. And that's how we learn about the history of cosmic expansion in our Universe. So what should you take away from this? That as light travels through space and space expands, it causes the wavelength of that very light to expand, too. That, among other things, is how we discovered dark energy and the accelerating Universe! Pretty remarkable stuff, and yet, not intuitive at all. It is from literally millions and millions of these individual measurements that we've been able to determine the entire history of how the Universe expanded. The redshift isn't hard to measure, either: Why? Well, if we measure the light from many, many distant objects and determine their distances, we can - simply based on the objects' redshifts - learn the entire history of how the Universe expanded. Expanding space causes a redshift! (And thanks to for the image!) You see, as space expands (above), the wavelengths of the light in it also expand, as you can see below.Īnd this last effect is so important for the expanding Universe. Remember, I told you that these distant galaxies aren't moving, the space between them is just expanding. One thing that's neat? If a light-emitting object moves towards you, the light gets blue-shifted, and becomes more energetic! (We see this happening for the Andromeda galaxy, one of the only ones in the Universe that moves towards us.) And although this is incredibly useful, this is not what's happening to light in the Universe. This is the same exact effect - the doppler shift - that causes police sirens to sound lower pitched when they move away from you. If an object that emits light moves away from you, the light from it gets redshifted. But gravitational redshift is rarely significant two other effects are far more important.Ģ. This is what we call redshift, where something happens to make the wavelength of your light longer and lower in energy. Smaller wavelength = higher energy and larger wavelength = lower energy, so if you need to climb out of a strong gravitational field, you lose energy, and therefore your light gets shifted towards the red. For light of all types, energy and wavelength are very closely related to each other. If you're deep in a gravitational field (like close to a black hole), you have to use up energy to climb out of it. Now there are three things - and only three things (unless you really want to get technical) - that can happen to this light to change the wavelengths that you see. For instance, if you have hydrogen, you'll always get light at wavelengths of 656 nanometers (red), 486 nm (cyan), 434 nm (indigo), 410 nm (violet), and 397 nm (on the border of violet/ultraviolet): You see, whenever an atom or molecule emits light, it gives off that light at a very few particular wavelengths. This brings us to my favorite application, which leads to the Hubble expansion: Last week, we began talking about understanding the size of the Universe, and we continued this week with some information on distances and motion in the Universe.
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