What's the Most Distant Galaxy? And Why Does It Matter?

"As a science communicator, I don't think a week goes by without a press release hitting my inbox informing me of astronomers finding some new record-breaking object. Sometimes it's the smallest planet yet discovered or the most iron-deficient star. But a very common claim is a distance record: the farthest galaxy from Earth ever seen, for example. When it comes to these sorts of record breakers, I have complicated feelings, built over decades of writing about them."

"Distance records are an excellent proxy for the state of the art in astronomy. Finding extremely faraway galaxies is hard. In general, objects get smaller and fainter with distance (although bizarre exceptions do sometimes apply), so huge telescopes are needed to spot them at all. Then comes the difficulty of actually determining their distance. We can't do this directly; it's not like we can hop onboard the starship Enterprise and keep our eyes on the odometer as we warp our way there."

"Light leaving a distant galaxy loses energy as it fights that expansion, so by the time it reaches us, its wavelength is stretched, which is what astronomers call a redshift. For historical (and mathematical) reasons, we say that a photon with its wavelength stretched out by a factor of two has a redshift of one; if the wavelength is three times longer, the redshift is two."

Distance-record announcements appear frequently and require careful interpretation. Some record claims are minor while others mark significant advances in observational capability. Extremely distant galaxies are generally smaller and fainter, necessitating very large telescopes to detect them. Direct distance measurement is impossible, so astronomers infer distance through redshift caused by cosmic expansion. Redshift measures wavelength stretching: a photon stretched by a factor of two corresponds to redshift one, by three corresponds to redshift two. Accurate distance claims therefore depend on both instrumental sensitivity and reliable redshift determination methods.

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