- 3964 - ASTRONOMY - gives us amazing science? - 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.
------------ 3964 - ASTRONOMY - gives us amazing science?
- 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, the
team measured a particular wavelength of light emitted by oxygen known as OIII.
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- 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 OII 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.
-
- 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.
-
- The rate of
cosmic expansion is 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.
-
- How long the
light traveled from the galaxy to reach us 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, 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? It turns out the galaxy is now about 32
billion light-years away.
-
- 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.
-
- The James
Webb Space Telescope has spotted some of the earliest and most distant
galaxies. Since it started delivering
its first results in July, 2022, the James Webb has been supplying stunning
images of the universe that include observations of the most distant and early
galaxies we've ever seen.
-
- Four of the
most distant and thus earliest galaxies known to date, which existed when the
13.8 billion-year-old universe was no more than around 350 million years old.
-
- And the
James Webb isn't alone in spotting distant and early galaxies. In December,
2022, the Atacama Large Millimeter/Submillimeter Array (ALMA) spotted faint
radio light from similarly ancient star groupings. This light began its journey
to us when the universe was less than 360 million years old.
-
- The link
between time and distance in space comes from the fact that light doesn't
travel instantaneously. Instead, light in a vacuum travels at a set speed of
around 3.0 x10⁸ meters per second.
-
- The sun is
about 93 million miles or around 8.33 light minutes from Earth, meaning that we
always see the sun as it was around 8 minutes and 20 seconds in the past. If
the sun suddenly blinked out, Earth wouldn't slip into darkness for around 8
minutes. An observer on Uranus, which is almost 20 times farther away from the
star than Earth, wouldn't notice for 2 hours and 40 minutes.
-
- This travel
time of light becomes really significant when we start to look out into the
universe with powerful instruments like the JWST, ALMA, and the Hubble Space
Telescope. The most distant galaxy
spotted by Hubble is GN-z11, which is located about 13.4 billion light-years
away.
-
- That means
the light has been traveling to us for over 13 billion years and Hubble lets us
see it as it was 300 to 400 million years after the Big Bang. A lot can happen
to this light during its billions of years crossing the universe to reach us,
and these effects are key to confirming the age and distance of these objects.
-
- The
universe isn't static, it is expanding and as this expansion progresses, the
space between galaxies stretches. Though
light is made up of particles called photons, it also has a wavelike nature.
Physicists refer to this dichotomy as "particle-wave duality.”
-
- Like all
waves, light has a wavelength — the distance between two "peaks"
along the wave — and a frequency — the number of times a peak passes a set
point every second — which are inversely proportional to each other and are
connected to the energy the light carries.
-
-
Long-wavelength light, say radio waves, has a low frequency and low
energy, while short-wavelength light, X-rays for example, has a high frequency
and high energy. The electromagnetic spectrum runs from long-wavelength light
like microwaves, radio waves and
infrared light to short-wavelength light like X-rays and gamma rays, the
most energetic form of light.
-
- The
expansion of the universe is causing galaxies to recede away from each other,
with more distant objects moving away more rapidly. This results in the
wavelength of light being "stretched" out toward the longer, redder,
frequencies of the electromagnetic spectrum. This is "redshift."
-
- The more
distant the galaxy, and thus the further back in time it is, the more shifted
its originally visible light is down to near-infrared or even infrared. This is
why the JWST views the universe in infrared and near infrared; it is the best
way to view very early and distant galaxies.
-
- There is an
important way of calibrating this method of measuring cosmic distances called
spectroscopy which also reveals information about these early galaxies.
Spectroscopy takes advantage of the fact that different chemical elements emit
and absorb light at a specific wavelength. That means light from distant
galaxies and stars carries the "fingerprint" of the elements they are
composed of.
-
- Scientists
look at the wavelength of light an element such an oxygen emits and absorbs in
a lab here on Earth and compare that with what they see in the universe. Oxygen
causes a feature in light called OIII at a wavelength of 88 micrometers here on
Earth. When ALMA looked at the spectra from a distant galaxy, astronomers saw
OIII at a longer wavelength, 1,160 micrometers meaning it had definitely been
redshifted.
-
- From the
difference in wavelength between the lab and the astronomical observation it's
easy to calculate the redshift. So for this ALMA galaxy z = (1160–88)/88 =
12.2, this means the galaxy has a redshift of 12.2. With some clever
mathematics, this value of z reveals that the galaxy is seen as it was when the
universe was just 360 million years old.
-
- The
chemical fingerprint of distant galaxies can be used to confirm the time period
at which galaxies are seen in another way.
The early universe was a sea of hydrogen and helium with just a
smattering of heavier elements, which astronomers call "metals" like
oxygen and nitrogen. These early stars in the first galaxies were
"metal-poor " as there just wasn't much in the way of heavy elements
for them to incorporate.
-
- The first
stars converted hydrogen to helium via nuclear fusion and then converted helium
to heavier elements, and when they exhausted this nuclear fuel, they could no
longer support themselves against the inward pressure of gravity. This
triggered the core of these stars to collapse, causing massive supernova
explosions that dispersed the forged heavy elements across their host galaxies.
-
- The remains
of these stars became the building blocks of the next generation of stars,
which was richer in metals than the prior generation. The process continues to
this day, creating stars and galaxies which are more metal-rich in every
generation.
-
- That means
astronomers can look at the spectra of light from a galaxy to see how abundant
in heavy elements it is to get a picture of the age of the universe at which it
is seen. If it's lacking in metals, that confirms it's probably a galaxy in the
early universe.
-
- Some cosmic
explosions from dying stars, Type Ia supernovas have a light output that is so
uniform that they are referred to as "standard candles." That means
looking at the redshift of spectra from these supernovas is a good way of
calculating distance across the cosmos, and thus helps confirm how distant some
galaxies are and how "recently" we are seeing them.
-
- 'Red shift'
is a key concept for astronomers. The term can be understood literally, the wavelength of the light is stretched, so
the light is seen as 'shifted' towards the red part of the spectrum.
-
- Light
behaves like a wave, so light from a luminous object undergoes a Doppler-like
shift if the source is moving relative to us. Ever since 1929, when Edwin
Hubble discovered that the Universe is expanding, we have known that most other
galaxies are moving away from us. Light from these galaxies is shifted to longer
(and this means redder) wavelengths - in other words, it is 'red-shifted'.
-
- Since light
travels at such a great speed relative to everyday phenomena (a million times
faster than sound) we do not experience this red shift in our daily lives.
-
- The red
shift of a distant galaxy or quasar is easily measured by comparing its
spectrum with a reference laboratory spectrum. Atomic emission and absorption
lines occur at well-known wavelengths. By measuring the location of these lines
in astronomical spectra, astronomers can determine the red shift of the
receding sources.
-
- However, to
be accurate, the red shifts observed in distant objects are not exactly due to
the Doppler phenomenon, but are rather a result of the expansion of the
Universe. Doppler shifts arise from the
relative motion of source and observer through space, whereas astronomical
redshifts are 'expansion redshifts' due to the expansion of space itself.
-
- Two objects
can actually be stationary in space and still experience a red shift if the
intervening space itself is expanding.
-
- The
behavior of light cannot be explained solely by models of particles or solely
by models of waves. If light is a particle, then why does it refract when
traveling from one medium to another? And if light is a wave, then why does it
dislodge electrons? But all behavior of light can be explained by combining the
two models: light behaves like particles and light behaves like waves.
-
- Particles
and waves are sometimes conceived as opposites, but they’re not. Light is not the only thing that exhibits
behavior of both particles and waves. Other quantum entities also have this
behavioral duality, and wave-particle duality is a key focus of the study of quantum
mechanics.
-
April 18, 2023 ASTRONOMY
- gives us amazing science? 3964
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