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Thursday, March 23, 2023
Dr. Brown: Henrietta Leavitt’s leading role in mapping the universe

We need maps, whether the maps embedded in our brain’s temporal and parietal lobes based on past experiences, tried and true paper maps or these days embedded in GPS devices in our cars and smartphones.

They all work. But what about mapping something as enormous as the universe? That’s the challenge scientists have taken on in the last two decades using the Sloan Telescope in New Mexico. The project, called the Sloan Digital Sky Survey, was designed to create the largest map of the universe to date by using gigantic clumps of galaxies as “standard candles” for determining distances in the universe.

Those who participated in last fall’s physics course at the Niagara-on-the-Lake Public Library might remember Henrietta Leavitt, a pioneering woman in a male astronomer’s world in the early 20th century.

Leavitt was tasked by her boss at the Harvard observatory with the job of identifying variable stars whose luminosities (brightness) varied in a cyclical fashion. She discovered that the periodicity with which variable stars brightened and dimmed was related to their apparent brightness, the brighter the star, the longer the cycle period and vice versa.

Once several variable stars with differing periodicities were found that were close enough to trigonometrically measure their distances from Earth, it was possible to calibrate the intrinsic luminosities of variable stars from their relative luminosities. That allowed scientists to determine the distance from Earth for variable stars too far away to measure their distances directly.

Leavitt’s variable stars became the first “standard candles” for measuring distances to faraway stars in the 1920s.

Edwin Hubble employed Leavitt’s technique to determine the distance to nearby nebulae, such as Andromeda, and showed that the latter nebula and others like it, were separate galaxies. Then, employing the doppler effect, Hubble went on to show, as Vasco Slipher had as early as 1912, that red-shifted nebulae were moving away from Earth and moreover, that the farther away those galaxies were, the faster they were moving.

Hubble’s observations were the first solid experimental evidence that the universe was expanding and led to the Big Bang hypothesis. For such important work, neither Hubble nor Leavitt won a Noble Prize, although Leavitt was considered for one but, unfortunately, she died before the Nobel committee made its decision.

Had Albert Einstein accepted the implications of his own field equations for general relativity in 1915, he could have won a second Nobel Prize because within months after its publication, it was apparent that his equations predicted an expanding universe, a view taken later by others such as Alexander Friedman and Georges Lemaitre.

Unfortunately, Einstein was so uncomfortable with the whole notion of an expanding universe that he inserted his famous cosmological constant into his field equations to correct for any apparent expansion of the universe. He was wrong and graciously acknowledged so in 1933 and shortly thereafter, withdrew his constant from his equations.

However, the story wasn’t over because later observations revealed the rate of the expansion of the universe was directly related to Einstein’s cosmological constant and the mathematical equivalent for what became known later in the 20th century as “dark energy.” Even wrong, I think Einstein would enjoy the twists and turns of his cosmological constant, first in, then out, then restored with new meaning.

These days the standard candles for mapping the universe use much larger targets – hangovers from the earliest universe. How so? We may not know what triggered the Big Bang but within a trillionth of a second or less, the universe expanded faster than the speed of light, ensuring that the universe would be more or less uniform in every direction.

Fortunately for us and the universe, not entirely uniform because those minor differences led early on to the gravitational clumping of dark matter (enigmatic matter which doesn’t interact with light) and ordinary matter (the stuff we’re familiar which early in the universe’s history, powerfully interacted with light).

The result was the formation of shell-like structures roughly 500 million light years in diameter, with dark matter forming the core and ordinary matter, forced outward by photons of light, forming the outer shell.

Gravity eventually compressed the ordinary matter in the outer shell enough to create galaxies and stars – and for those interested in mapping the universe – provided giant standard candles of more or less similar diameters, against which smaller (more distant) or larger (closer) shells could be compared to calculate their distances relative to Earth.

Henrietta Leavitt would be proud to learn that those who map the universe in modern times still use standard candles to measure distances to faraway star systems in much the same manner as she pioneered using variable stars a century earlier. As astronomers worldwide continue to do, so too should we, tip our hats to the female astronomer who started it all.

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the Infohealth series at the Niagara-on-the-Lake Public Library.  


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