Across the Universe

In Cosmology by Brian Koberlein11 Comments

Astronomers have discovered what seems to be the most distant galaxy yet discovered. The galaxy known as EGS-zs8-1 has a redshift of z = 7.73, trumping the previous record for a galaxy at z = 7.51. Several articles are giving a distance of this galaxy as about 13 billion light years away, but that’s not really an accurate measure, and it’s part of the reason we usually talk about redshift rather than distance.

The redshift of a galaxy is typically given by a number known as z, which is the difference between the observed wavelength of a particular emission line and the standard wavelength as measured here on Earth, divided by the standard wavelength. In this way, an object with no redshift would have a z = 0, and the greater the redshift the greater the z. The nice thing about redshift is that it’s purely an observational result.

From this z number we can infer a distance based upon Hubble’s law, which states that the greater the redshift the more distant the galaxy. But since Hubble’s law also means the universe is expanding, we have to be careful about which distance we mean. Do we want the distance of the galaxy when the light left it? The distance the light traveled to get to us? Or the distance of the galaxy now? These are all different.

With a z of about 7.73, that means the galaxy was about 3.4 billion light years away when the light left it. Because of the expansion of the space between us and the galaxy, it took the light about 13 billion years to reach us. But since then the galaxy has moved away from us at an ever greater rate, so it is now about 29.5 billion light years away from us.

That last distance might seem odd given that the universe is only 13.7 billion years old, but it’s important to keep in mind  that the galaxy hasn’t traveled 26 billion light years in 13 billion years. In fact any actual motion away from us through space would be in addition to the 26 billion light years. That last distance is entirely due to the expansion of space between us. In an expanding universe, distance is always changing.

Which is why astronomers tend to stick with redshift.

Paper: P. A. Oesch et al. A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE. ApJ 804 L30 (2015)

Comments

  1. Can you post a graph or a table that converts between z factor and the distances (comoving, coangular and current)?

    Also, is this influenced by the distribution of dark matter and dark energy, so depends on the direction of the object?

    Thanks for your great work on this blog.

  2. Um, a z of 0 is ‘local’, or here. Redshift is defined as the ratio of observed wavelength over (divided by) ‘restframe’ wavelength minus 1.

    @ Amir: Ned Wright’s CosmoCalc is perhaps the best (google it); in the geometry of the universe – which is based on General Relativity – ‘distance’ is no longer simple, there are several different ‘distances’, for the same galaxy!

  3. All that’s said above makes me think that Ruben Dario was not too far off when he said : “nada es verdad o mentira, todo depende del cristal con que se mira…”

    1. Hi Ramiro, this quote is quite apt in that everything does depend on the color of this distant galaxy. Astronomers can often deduce a galaxy’s distance from Earth by very carefully measuring its color in visible and infrared light.
      Astronomers were able to identify the galaxy EGS-zs8-1 as a high redshift candidate in survey images taken by the Hubble and Spitzer space telescopes. Considering color alone it appeared to be one of the brightest and most massive objects found by astronomers in the early universe. However, sometimes color can be deceptive. Perhaps EGS-zs8-1 is really a peculiar red galaxy with a much lower redshift.
      To confirm the high redshift and take the current distance record, the galaxy needs to have its redshift measured by spectroscopy. This is why some of the very distant galaxies seen in Hubble’s Frontier Fields survey (thanks to gravitational lensing) do not hold the record even though their redshift may be z~8-1z~10. No one has yet been able to confirm the redshift estimate that was obtained by studying images from HST.
      Spectroscopy of the early Universe is no easy task. Both Hubble and Spitzer are equipped to carry out spectroscopic measurements but their instruments are not sensitive enough to target extremely distant faint galaxies seen at very early cosmic times. This is why astronomers are patiently waiting for the new James Webb Space Telescope, which will have the capabilities to carry out these very challenging measurements.
      However, in this particular case the astronomers were able to successfully confirm the high (record-breaking) redshift of EGS-zs8-1 , by using a relatively new, powerful spectrometer called MOSFIRE (Multi-Object Spectrograph for Infrared Exploration) on the Keck 10 meter Telescope in Hawaii. MOSFIRE has a sensitive state-of-the-art detector and electronics system, which allowed the astronomers carrying out this research to make observations of this distant galaxy and make the z~7.7 measurement.
      I think the complete quatrain Ramiro quotes says:
      “In this treacherous world, nothing is truth or lie, everything depends on the color of the glass through which you look “.
      I’d like to amend this text if we’re applying it to the Universe:
      “In this treacherous Universe, nothing is truth or lie, everything depends on the sensitivity of the spectrometer that is attached to the telescope through which you look”.

  4. If, as big bang theory states, at the moment of the big bang, we were right next to the contents of EGS-zs8-1, and now, 13.7 billion years later, we are 26 billion light years away, then RELATIVE TO US, the galaxy has receded 26 billion light years in 13.7 billion years, thereby receding from us at twice the speed of light. This means that current theory considers things to accelerate with the space in which they are located. Is this correct? If not, are you able to resolve the paradox?

    1. Author

      It’s not that things accelerate with the space their in, but rather that the space between objects expands over time, giving the relative motion. It might seem odd, but keep in mind in general relativity space is not a fixed absolute, but can be warped and stretched.

      1. Thanks very much for the reply, I appreciate your time. Sorry I did’t spell out the paradox very clearly. The paradox is that the light emitted at the big bang was right next to us when it was emitted. Now it arrives, 13.7 billion years later, having been emitted right next to us all that time ago. A reformulation of Hubble’s law would be stronger under Occam’s razor than inflation.

  5. In some news in the internet I’ve found the message regarding the brightest galaxy ever seen – WISE J224607.57-052635.0. All news says that the galaxy is 12.5 billion light years away. Is this the same situation as you have described in the text above? That means the light needs 12.5 billion years to reach the earth and the current distance is much larger?

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