Gamma-ray bursts can help astronomers measure great distances across the universe

Now that the James Webb Space Telescope is up and running, astronomers can study some of the faintest and most distant galaxies ever seen. By some accounts, we may have already captured an image of a galaxy from when the universe was only 300 million years old. But we can’t be entirely sure of its distance, and that’s a big problem for astronomers.

How do you measure the distance to the farthest galaxy? If you are an astronomer, you rely on its observed redshift. Since the universe is expanding, the further away the galaxy is, the more its light is redshifted. To calculate the galaxy’s distance, astronomers plug the redshift into a formula derived from the standard cosmological model. Observing everything from variable stars to distant supernovae, we know very well the relationship between redshift and distance. So do the math and calculate the distance.

Of course, the most distant galaxies are further away than the Standard Model calibration observations. We can extrapolate the Standard Model for these galaxy distances, but that assumes that the cosmic acceleration was not drastically different at the time. There’s no reason to assume our hypothesis is wrong, but it would be nice to get some distance measurements to the edges of the Webb galaxies. Unfortunately, the type of supernova we use for these distance measurements is not bright enough to be seen at this distance. But a new study reveals that we could use gamma-ray bursts instead.

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The cosmic distance scale for measuring galactic distances.
The cosmic distance scale for measuring galactic distances. Credit: NASA, ESA, A. Feild (STScI) and A. Riess (STScI/JHU)

Gamma-ray bursts, or GRBs, are short-lived bursts of gamma radiation. They are probably caused by hypernova explosions of giant stars, and they are extremely powerful. A single GRB releases more energy than the Sun will emit during its entire 10 billion year lifespan. But the light curves of GRBs are complex, making it difficult to find a common pattern.

In this latest study, the team did not look at variation in gamma light, but rather optical light. They examined the optical light curves of 500 known GRBs and found nearly 180 with a common pattern. Many of these GRBs are close enough that we know their distances, so the team was able to use their observed luminosities to calculate the actual luminosities. This means that if a very distant GRB of this common type is detected, astronomers can use its observed brightness to calculate its distance, thereby extending our cosmic distance scale. In time, we may be able to test our Standard Model hypothesis with this technique.

So while everyone is rightfully excited about the discoveries from the James Webb Space Telescope, it’s good to know that astronomers are also working on tools to support the discoveries Webb will make.

Reference: Dainotti, Maria Giovanna, et al. “Two-dimensional and three-dimensional fundamental-plane optical correlations for over 180 gamma-ray burst afterglows with Swift/UVOT, RATIR, and the SUBARU telescope.” The Astrophysical Journal Supplementary 261 (2022): 25.

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