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Skywatching

Solutions sought to measuring speed and distance travelled by planets in space

Trouble with Hubble

Our knowledge of the expansion of the universe is fundamental to understanding how it formed and how it has and is evolving.

That expansion was first measured by Edwin Hubble. He compared measurements of how far away galaxies lie with the speeds they are receding from us. The relationship between the distance and the speed of recession is known as Hubble's Constant. We have a problem with chaining down the precise value of Hubble's Constant and hoped observations made using the James Webb Space Telescope would resolve the issue. However, unfortunately, it has made things worse.

It is fairly easy to measure the speed with which a distant galaxy is receding (or approaching). We use spectroscopes to detect the signatures of particular elements in the light from that galaxy. If the object is receding from us, we will see those signatures shifted to longer wavelengths, that is, reddened. The amount of the reddening, or "the red-shift", gives us the speed.

Getting distances is a bit harder. Fortunately there is a class of variable stars called cepheids. These stars cycle in brightness, and the duration of each cycle is related to the average brightness of the star. We search images of distant galaxies for cepheids, and then measure how long each cycle of brightness changes takes. We can then calculate the light energy output (the luminosity) of the star and compare that with the observed brightness.

That will tell us how far away it is. If we measure the brightness of a distant light, and know that it is a 100-Watt light, we can calculate how far away it is. So by searching for thousands of cepheids lying in distant galaxies, and measuring their red-shift, we can evaluate Hubble's Constant and calculate how long ago everything in the universe was in one lump. Measurements made using the Hubble Space Telescope have been a mainstay of this work. The value obtained for Hubble's Constant was about 74 kilometres a second per million parsecs. That is, for each additional million parsecs of distance, the recession speed of galaxies as they are carried away by the expansion of the universe increases by 76 kilometres a second. A parsec is a unit of distance, and is equal to about 3.26e13 km.

There is another method for determining Hubble's Constant. That involves looking at the tiny irregularities in the cosmic microwave background, the fading breath of the Big Bang. That originated 380,000 years after the beginning, when, for the first time the universe cooled enough for atoms to come together, allowing light to travel through it. That method is also believed reliable, but gives a different answer—67 kilometres a second per million parsecs.

This discrepancy was widely believed to be measurement errors, and more precise determinations would resolve the issue.

That is where the James Webb Space Telescope came in. However, a long series of measurements yielded an answer that agreed well with the measurements made using the Hubble Space Telescope, So, that discrepancy between the results obtained using the two different methods of measurement is real. Could it be that the two sets of measurements are not really describing the same thing?

Many possibilities are being looked at. One of them is a revival of an idea suggested a long time ago to explain the reddening of the light from distant galaxies. The idea is, as it travels over cosmic distances, light loses energy, it gets "tired". When light loses energy it becomes redder. That may explain why the measured red-shifts are higher than expected. There are other possibilities, so the next year or two should be interesting.

•••

• Venus and Mars lie extremely low in the dawn glow.

• Jupiter shines in the west after sunset, with Mercury hiding low in the sunset glow.

• The Moon will reach its last quarter on April 1.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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