Cosmology FAQ

This cosmology FAQ answers questions that I've actually been asked on various forums on the Web.

Q1: "Where is point zero in the universe? At what velocity are we relative to point zero? In which direction do we accelerate to increase our velocity? How close are we to the speed of light? Is this theoretically even possible to calculate? Maybe the speed of light is not that far away relative to point zero?"

A1: It is not clear what is meant by "point zero", but the closest thing I can think of is a point at rest w.r.t. the cosmic microwave background (CMB) radiation. Our part of the universe (our local group of galaxies) moves at about 600 km/s or about 0.002c relative to the CMB. This velocity seems constant - if we are accelerating, it is too small to be detected at present.

If however, "point zero" meant "center of the Big Bang (BB)" or "center of the cosmos", the answer is simple: there is no such thing as a point that represents the BB or the center of the cosmos. The BB happened everywhere at once.

Q2: If there is no absolute reference in the universe, how can there be a speed limit?

A2: The 'speed limit' refers to relative speed. Einstein predicted and experiment confirmed that however hard we try, we will not succeed in 'boosting' anything to even reach the speed of light relative to us, never mind exceeding it. For this to be true, we must be at rest in one inertial frame of reference, or course.

Q3: How do we know that the Cosmic Microwave Background (CMB) does not define an absolute frame of reference, the only one in which light propagates at "c"?

A3: As Earth orbits the Sun at 30 km/s, our velocity relative to a given spot in the CMB, lying near the ecliptic, changes periodically with a peak-to-peak value of 60 km/s. We can measure this velocity change by means of the redshift change of the CMB.

We are however unable to detect a change in the speed of any light coming from that general direction, e.g., from pulsars. This indicates beyond reasonable doubt that the measured speed of light is independent of our movement relative to the CMB.