Measuring speed of universe’s expansion not easy
You’ve probably hear it many times — the universe is expanding.
A harder question, at least for astronomers, is how fast is the universe expanding?
That speed is difficult to measure precisely. Several techniques are used to get the distance and speed of distant objects, but not all of these techniques agree with one another.
One method that uses the heat of radiation, or cosmic microwave background, left over from the Big Bang indirectly determines the expansion speed. This method uses a mathematical model of the expanding universe that includes mysterious “dark energy.”
Assuming this model is correct, it gives a precise value of the Hubble constant, or the speed of expansion.
Another method is a direct measurement of the expansion, but that is more difficult to perform.
This method involves observations of certain types of stars, called Cepheid variables, and also supernovas in distant galaxies. Astronomers have methods to determine the brightness of both of these at their source, and by comparing these to the light level measured by telescopes, the distance can be calculated.
It’s like using the light in your eye from a distant light bulb and knowing whether it’s a 60-watt or 100-watt bulb at the source to estimate the distance.
There is a new method to measure the Hubble constant, and it uses a technique called gravitational lensing, which was first predicted using Einstein’s equations for gravity.
Maybe you’ve heard that light bends near massive objects such as black holes. Also, it’s been known for centuries that light bends when it enters glass, such as in the lens of a magnifying glass.
That means light from distant objects, such as quasars, can bend when traveling past a massive galaxy. The massive galaxy acts as a lens and bends the quasar light to a focal point.
This forms multiple images in a telescope on Earth if the quasar and the galaxy are lined up just right. Such a result of gravitational lensing is shown in the picture accompanying this column. In it, four images of a quasar surround the galaxy in the center.
Using gravitational lensing, a team of astronomers called the H0LiCOW collaboration (as in holy cow, Batman!) announced earlier this year a direct measure of the Hubble constant.
The key result is that H0LiCOW agrees with the Hubble constant found by the other direct measurements and disagrees with that found from the cosmic microwave background.
So what’s going on? After all, we have only one universe, and its speed of expansion is fixed by nature. Obviously, one (or more) of the measurement techniques is flawed.
In my view, the most likely explanation is that the mathematics used to describe the dark energy in the cosmic microwave measurements is incorrect. There are other models proposed by theoretical physicists that put the effect of dark energy into the equations in a different way, so maybe one of these alternate models is correct.
The bottom line is that there are some details about the expansion of the universe that we still don’t understand. All measurements agree that the universe is expanding, but whether the Hubble constant has a value of 66 or 73 (in the appropriate measuring units) is still an issue.
While this might seem a small difference between the different ways to measure the speed of expansion, precise measurements have the effect of telling us whether or not we have a correct physical model of the universe.