Anonymous asked: In your post "post/21800557534/if-the-universe-is-expanding-does-that-mean-that" you stated that "Astronomers commonly probe the depths of space that reveal redshifts larger than one (z>1) which suggests that these observed objects are receding at greater speeds than that of light." If, for example, distant stars are receding faster than the speed of light, then how do the photons they emit reach us? Or are astronomers probing the depths of space without the aid of photons?
Very good question! This startled me at first too, and I’m glad you picked up on this. It was not a mistake; for example, the Cosmic Microwave Background radiation had a temperature of approximately 3,000K when it was emitted and measurements suggest it has a current temperature of ~2.725K. For simplicity, let’s say the current CMB temperature is 3K. The CMB, then, had been stretched by a factor of 1,000 and thus had been redshifted by about 1,000. To merely observe the CMB radiation which happened when the universe was about 400,000 years old is to probe so far into space that the redshift is as high as 1,000.
Now, this is where your confusion arises; does this mean that objects as far as the CMB are moving 1,000 times the speed of light? Is the CMB that we detect moving at 1,000 times the speed of light? Well, no. There’s two different types of redshifts that can be measured: the real motion of objects (which is given by the Doppler Shift of the motion of objects in the radial direction which we can measure from their emission spectra) and then there’s measuring the expansion rate of the Universe itself. At low redshifts, the measurements are significantly due to the real motion of objects, but at high enough redshifts, especially redshifts as high as 1,000, that is certainly a measurement of the expansion rate of the Universe itself.
Though objects such as galaxies cannot move faster than light, the expansion rate of the Universe is an exception. In my first year of my undergraduate studies, what I’m about to tell you was by far the coolest thing I learned: on larger scales, nothing is actually moving. Distant galaxies and their spectra suggest that they are moving away from us (that’s what their redshifts tell us), but in reality, it’s actually the expansion of the Universe that is causing their redshifts. It is the space BETWEEN GALAXIES that is expanding, and while photons are traveling within this space, these photons get stretched by the expansion of the Universe itself which makes them look red. Thus, though we talk about recessional velocities of distant galaxies, it’s actually the expansion rate of the Universe that we’re measuring, which is allowed to exceed the speed of light. So, the CMB isn’t receding at 1,000 times the speed of light… the Universe at that point is expanding at 1,000 times the speed of light.
Thank you for this wonderful question. I always love explaining this to people.