- 4125 - REDSHIFT - tell us distance to galaxies? Astronomers can't directly measure the distance of galaxies billions of light years away. Instead, they measure what is known as “redshift”, or “z”. In this case, they measured a particular wavelength of light emitted by oxygen known as “OIII”.
-------------- 4125 - REDSHIFT - tell us distance to galaxies?
- December 2022, astronomers using the
“Atacama Large Millimeter/submillimeter Array” (ALMA) confirmed the discovery
of one of the most distant galaxies ever observed. The faint radio light ALMA
captured began its journey to us when the universe was less than 360 million
years old.
-
- It's a tremendously distant galaxy, but
just how far away is it really? The answer is a bit complicated, and it depends
on what you mean by “distance”.
-
- Astronomers can't directly measure the
distance of galaxies billions of light years away. Instead, they measure what
is known as “redshift”, or “z”. In this case, they measured a particular
wavelength of light emitted by oxygen known as “OIII”.
-
- When we observe the OIII emission line in a
lab here on Earth, it has a wavelength of 88 micrometers. The OIII line ALMA
observed in this particular galaxy was much longer, about 1,160 micrometers.
-
- Since red light has a longer wavelength
than blue light, we say the observed OIII line is shifted to the red, or
redshifted. Given these two numbers, calculating z is easy. It is just the
relative redshift of the observed light, So z = (1160—88)/88 = 12.2 The bigger
the z, the greater the redshift, and z = 12.2 is the largest confirmed redshift
of a galaxy so far.
-
There are
two ways light from a galaxy can be redshifted. The first is known as the
“Doppler shift” and is caused by the physical motion of a galaxy through space.
You're familiar with this effect in sound. When a train speeds past you, its
horn sounds higher as the train approaches you and lower as it passes you and
rolls away. The sound waves are bunched up as the train moves toward you and
have a higher pitch, and they are stretched out as the train moves away from
you, thus a lower pitch.
-
- The same thing happens with light. If a
galaxy is moving toward us its light is blueshifted, and the light is
redshifted if it's moving away from us.
-
- The second way redshift can occur is
through cosmic expansion. The universe is expanding, and this means as light
travels to us from a distant galaxy its wavelength is stretched out by the
expansion of space. The longer the light travels the more the light is
stretched, so the more the light is redshifted.
-
- This is known as “cosmological redshift”.
For distant galaxies, almost all the redshift we observe is cosmological. This
is how we know high redshift galaxies such as this one are very, very far away.
-
- But this still doesn't tell us the specific
distance. To determine that we have to look at how the universe expands over
time. Right now there's a bit of uncertainty about the rate of cosmic
expansion, known as the “Hubble parameter”. The Planck mission observations of
the cosmic microwave background put the value at about 68 (km/s)/Mpc, while
observations of the Hubble and Gaia spacecraft give it a higher value of about
72 (km/s)/Mpc.
-
- The bigger the value, the faster the universe
is expanding and the farther away distant galaxies are. If we pick a middle
value of 70 (km/s)/Mpc, then we can calculate a reasonable distance using
general relativity, but even then our answer will depend on how we define
distance.
-
- One definition would be to ask how long the
light traveled from the galaxy to us. This is known as the “light travel time”
and turns out to be about 13.1 billion years. Since the universe is about 13.46
billion years old (based on the Hubble parameter we chose), that means the
light left the galaxy when the universe was about 360 million years old.
-
- Since the light traveled for 13.1 billion
years, does that mean the galaxy is 13.1 billion light-years away? Not quite.
Because of cosmic expansion, the light traveled for much longer than it would
have if the universe wasn't expanding. The galaxy was closer to us when the
light began its journey. Much closer.
-
- If we calculate how far away the galaxy was
from us 13.1 billion years ago, we get 2.4 billion light-years. So this galaxy
was only 2.4 billion light years away, but the universe expanded so much that
its light took 13.1 billion light-years to reach us.
-
- The galaxy was 2.4 billion light years away,
but once the light started heading our way the galaxy continued to move away
from us because of the ever-expanding universe. So where is the galaxy now?
-
- If you do the math, it turns out the galaxy
is now about 32 billion light-years away. But wait a minute? How can we see a
galaxy 32 billion light-years away if the universe is less than 14 billion
years old? The answer is that we can't. ALMA's view of the galaxy is how it
looked when it was only 2.4 billion light-years away.
-
- We will never be able to see what the
galaxy looks like now. It is too far away, and the universe is expanding too
quickly for that light to reach us. We only see the optical echo of where it
was and how it used to appear.
-
- We usually talk about how old the universe
was when the galactic light began its journey. That's enough to tell us that
the galaxy is far away and seen from long ago. So long ago and so far away that
its distance is hard to define.
-
-
August 11, 2023 REDSHIFT
- tell us distance to
galaxies? 4125
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