How old is the Universe? (2)

Following on from the previous post I will first recap the state of our knowledge about the Universe at the beginning of the 20th century. We knew the distance to the closest stars outside the Solar System (using parallax) and also realised that many distant stars are moving away from us (because their light spectrum is redshifted). But we had no idea if the distant stars were thousands, millions or billions light years away from the Earth.

The problem with determining distances to visible stars is that we can only assess their apparent brightness (how bright they appear to us) but not absolute luminosity (the amount of light they emit). On a foggy night a 50W light bulb close to us may appear just as bright as a 100W bulb further away and unless we know their wattage we will not be able to assess the true distance. What was needed was a “standard candle” – stars of known luminance which would serve as yardsticks of the Universe. They would be like 100W light bulbs at various spots whose apparent brightness would (inversely) correlate with the distance from us.

Cepheids are variable stars whose apparent brightness changes cyclically over time – they are like bulbs turned periodically up and down. In early 20th century Henrietta Leavitt, through painstaking observation, identified a number of cepheids in one nebula known as Magellanic Clouds. The variable stars she catalogued had different apparent brightness and different periods (the length of time they took to go through the dimming/brightening cycle). She then made an inspired guess that a relationship may exist between their period and luminosity. Since all these cepheids were located in one group of stars their relative distances to Earth were not a factor in assessing absolute luminance from apparent brightness. In 1912 Henrietta Leavitt published the following graph which plotted the maximum and minimum apparent brightness for the 25 cepheids in her study (vertical axis) against their period (horizontal axis):


The graph, with time drawn in log scale, shows a remarkably consistent relationship – the brighter a cepheid, the longer its period. This was like finding out that stronger bulbs were turned up and down less frequently. So if, for example, we saw two blinking lights in fog we would know that the one pulsating more frequently has say half of the wattage of the other one. We would still not be able to tell the distance, though – they could be a pair of 5W/10W bulbs close up or 50W/100W further away. But if we  somehow managed to measure the distance to one of them it would become a reference for locating all blinking bulbs. Comparing the period of any bulb to the reference one would tell us its wattage. Knowing the wattage and its apparent brightness we could work out the distance.

Luckily, 13 of Miss Leavitt’s cepheids were close enough to Earth for their distances to be measured using parallax. A standard candle of the Universe was finally identified. In early 1920s Edwin Hubble found more cepheids in other nebulae and measured distances to them, using the known relationship between period and luminosity. His results, published in 1924, stunned the World.

How old is the universe? (3)


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