The expanding Universe
Ken Tapping, September 11, 2018
The universe is expanding. It started to expand at the Big Bang, almost 14 billion years ago. We know how fast it is expanding and that it is accelerating. We owe our knowledge to three instruments: The first is the telescope. Distant objects can be extremely faint, and the biggest possible light collector is needed to catch enough light to make an image or to analyze. For example the 3.6m diameter mirror in the Canada France Hawaii Telescope (CFHT), which is now by no means the largest telescope in the world, collects more than 50 million times as much light as our unaided eyes. In addition to making images, the light collected by the telescope can be fed to two devices. One is the photometer, a device for measuring precisely the apparent brightness of cosmic objects. The other is the spectrometer, which is used to measure the composition of the light. In addition, the universe is so big that looking further and further away is looking further and further back in time. If we look at an object a billion light years away, we see it as it was a billion years ago, when its light started on its way to us. By looking at more and more distant objects we can read the history of the universe, providing we know how far away the objects are. Then, to study the expansion of the universe we need to be able to know how fast objects are receding from us.
To find distances we measure the apparent brightnesses of objects. If we know their energy output – their luminosity – we can then calculate how far away they are. If you know that distant point of light is a 100W bulb, you can measure its apparent brightness and then estimate its distance. For nearby galaxies there are stars such as Cepheids and RR-Lyrae variables, which cycle in brightness in a manner related to their luminosity. For more remote objects our main yardstick is a type of exploding star. The explosions are produced by a kind of binary star, where one of the pair is a white dwarf, an Earth-sized relic with no fuel left, still very hot but very slowly cooling off. It pulls material off its partner, which accumulates on its surface. When a critical amount has been collected, a runaway fusion reaction takes place, like a supersized hydrogen bomb. Because the explosion happens when a critical amount has been collected, we can estimate the energy output. When one of these explosions occurs in a distant galaxy, we measure how bright it looks, which gives us its distance.
The last tool in our analysis of the universe's expansion is the Doppler effect. We've all heard it. For example, when a noisy motorcycle passes by, or we watch a train passing us at a level crossing, the sound has a higher pitch when the source is approaching, and a lower tone when it is receding. The same applies to light. If the source is approaching us, its light is shifted to the blue; if it is moving away, its light is reddened, or redshifted. All the elements have unique multicolour signatures, or spectra. For example, in the laboratory we can measure the spectrum of hydrogen, a common element in the universe. Then we search for the spectral signature of hydrogen in distant, cosmic objects. By comparing the two spectra we can determine how much the cosmic spectrum is red-shifted, which tells us how fast the object is receding. If we have the distance and the redshift for an object, and noting that distance is linked to time, we know the speed the universe was expanding at that point in the past. By measuring the redshift of objects at different distances, we can measure how fast the universe is expanding at different points in its history. We would expect the expansion after the initial Big Bang to be slowing down. Instead, surprisingly, it is speeding up. We cannot yet explain this.
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