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Galaxy distribution

To determine the distribution of galaxies in three-dimensional space, astronomers have to measure their positions and their redshifts. The larger the volume of space surveyed, the more likely the measurement is a fair sample of the universe as a whole. However, the work involved increases very rapidly as you increase the volume covered by the survey.

Let’s do a quick calculation to see why this is so.

Suppose that you have completed a survey of all the galaxies within 30 million light-years and you now want to survey to 60 million light-years. What volume of space is covered by your second survey? How much larger is this volume than the volume of your first survey? Remember that the volume of a sphere, V , is given by the formula V = 4/3π R 3 , where R is the radius of the sphere.

Solution

Since the volume of a sphere depends on R 3 and the second survey reaches twice as far in distance, it will cover a volume that is 2 3 = 8 times larger. The total volume covered by the second survey will be (4/3)π × (60 million light-years) 3 = 9 × 10 23 light-years 3 .

Check your learning

Suppose you now want to expand your survey to 90 million light-years. What volume of space is covered, and how much larger is it than the volume of the second survey?

Answer:

The total volume covered is (4/3)π × (90 million light-years) 3 = 3.05 × 10 24 light-years 3 . The survey reaches 3 times as far in distance, so it will cover a volume that is 3 3 = 27 times larger.

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Even larger, more sensitive telescopes and surveys are currently being designed and built to peer farther and farther out in space and back in time. The new 50-meter Large Millimeter Telescope in Mexico and the Atacama Large Millimeter Array in Chile can detect far-infrared and millimeter-wave radiation from massive starbursting galaxies at redshifts and thus distances more than 90% of the way back to the Big Bang. These cannot be observed with visible light because their star formation regions are wrapped in clouds of thick dust. And in 2018, the 6.5-meter-diameter James Webb Space Telescope is scheduled to launch. It will be the first new major visible light and near-infrared telescope in space since Hubble was launched more than 25 years earlier. One of the major goals of this telescope is to observe directly the light of the first galaxies and even the first stars to shine, less than half a billion years after the Big Bang.

At this point, if you have been thinking about our discussions of the expanding universe in Galaxies , you may be wondering what exactly in [link] is expanding. We know that the galaxies and clusters of galaxies are held together by their gravity and do not expand as the universe does. However, the voids do grow larger and the filaments move farther apart as space stretches (see The Big Bang ).

Astronomy and technology: the sloan digital sky survey

In Edwin Hubble’s day, spectra of galaxies had to be taken one at a time. The faint light of a distant galaxy gathered by a large telescope was put through a slit, and then a spectrometer (also called a spectrograph) was used to separate the colors and record the spectrum. This was a laborious process, ill suited to the demands of making large-scale maps that require the redshifts of many thousands of galaxies.

But new technology has come to the rescue of astronomers who seek three-dimensional maps of the universe of galaxies. One ambitious survey of the sky was produced using a special telescope, camera, and spectrograph atop the Sacramento Mountains of New Mexico. Called the Sloan Digital Sky Survey (SDSS), after the foundation that provided a large part of the funding, the program used a 2.5-meter telescope (about the same aperture as the Hubble) as a wide-angle astronomical camera. During a mapping program lasting more than ten years, astronomers used the SDSS’s 30 charge-coupled devices (CCDs)—sensitive electronic light detectors similar to those used in many digital cameras and cell phones—to take images of over 500 million objects and spectra of over 3 million, covering more than one-quarter of the celestial sphere. Like many large projects in modern science, the Sloan Survey involved scientists and engineers from many different institutions, ranging from universities to national laboratories.

Every clear night for more than a decade, astronomers used the instrument to make images recording the position and brightness of celestial objects in long strips of the sky. The information in each strip was digitally recorded and preserved for future generations. When the seeing (recall this term from Astronomical Instruments ) was only adequate, the telescope was used for taking spectra of galaxies and quasars—but it did so for up to 640 objects at a time .

The key to the success of the project was a series of optical fibers , thin tubes of flexible glass that can transmit light from a source to the CCD that then records the spectrum. After taking images of a part of the sky and identifying which objects are galaxies, project scientists drilled an aluminum plate with holes for attaching fibers at the location of each galaxy. The telescope was then pointed at the right section of the sky, and the fibers led the light of each galaxy to the spectrometer for individual recording ( [link] ).

Sloan digital sky survey.

Panel (a), at left, shows the open aperture of the SDSS telescope in New Mexico. The primary mirror is clearly seen at the base of the telescope. Panel (b), at right, shows Richard Kron inserting optical fibers into a pre-drilled plate in preparation for taking the spectra of many galaxies simultaneously.
(a) The Sloan Digital Sky Survey telescope is seen here in front of the Sacramento Mountains in New Mexico. (b) Astronomer Richard Kron inserts some of the optical fibers into the pre-drilled plate to enable the instruments to make many spectra of galaxies at the same time. (credit a, b: modification of work by the Sloan Digital Sky Survey)

About an hour was sufficient for each set of spectra, and the pre-drilled aluminum plates could be switched quickly. Thus, it was possible to take as many as 5000 spectra in one night (provided the weather was good enough).

The galaxy survey led to a more comprehensive map of the sky than has ever before been possible, allowing astronomers to test their ideas about large-scale structure and the evolution of galaxies against an impressive array of real data.

The information recorded by the Sloan Survey staggers the imagination. The data came in at 8 megabytes per second (this means 8 million individual numbers or characters every second). Over the course of the project, scientists recorded over 15 terabytes, or 15 thousand billion bytes, which they estimate is comparable to the information contained in the Library of Congress. Organizing and sorting this volume of data and extracting the useful scientific results it contains is a formidable challenge, even in our information age. Like many other fields, astronomy has now entered an era of “Big Data,” requiring supercomputers and advanced computer algorithms to sift through all those terabytes of data efficiently.

One very successful solution to the challenge of dealing with such large datasets is to turn to “citizen science,” or crowd-sourcing, an approach the SDSS helped pioneer. The human eye is very good at recognizing subtle differences among shapes, such as between two different spiral galaxies, while computers often fail at such tasks. When Sloan project astronomers wanted to catalog the shapes of some of the millions of galaxies in their new images, they launched the “Galaxy Zoo” project: volunteers around the world were given a short training course online, then were provided with a few dozen galaxy images to classify by eye. The project was wildly successful, resulting in over 40 million galaxy classifications by more than 100,000 volunteers and the discovery of whole new types of galaxies.

Key concepts and summary

Counts of galaxies in various directions establish that the universe on the large scale is homogeneous and isotropic (the same everywhere and the same in all directions, apart from evolutionary changes with time). The sameness of the universe everywhere is referred to as the cosmological principle. Galaxies are grouped together in clusters. The Milky Way Galaxy is a member of the Local Group, which contains at least 54 member galaxies. Rich clusters (such as Virgo and Coma) contain thousands or tens of thousands of galaxies. Galaxy clusters often group together with other clusters to form large-scale structures called superclusters, which can extend over distances of several hundred million light-years. Clusters and superclusters are found in filamentary structures that are huge but fill only a small fraction of space. Most of space consists of large voids between superclusters, with nearly all galaxies confined to less than 10% of the total volume.

Questions & Answers

something's like that maybe, the ways to know them
Dennis Reply
oh ok you can actually reply to your own comment by tapping it once
Tanmay
you should do this
Tanmay
what are laws of the universe
Dennis Reply
The law of attraction
Lambent
Are you talking about fundamental forces Dennis?
Tanmay
you can go to google and type 4 fundamental forces of the universe
Tanmay
let me give you gist of it there are 4 fundamental forces namely gravitational, electromagnetic, strong nuclear and weak nuclear
Tanmay
the came into being just after the bid bang
Tanmay
I mean big bang
Tanmay
you should watch this video for better understanding ***youtu.be/X9otDixAtFw
Tanmay
wow some more than information
Rajeshwari Reply
since space gets bigger dose it affect distance between the sun and earth
Swazi Reply
yes the distance between earth and sun increase at a rate of 15 cm per year
Tanmay
ohh really I didn't knew that , you learn something new everyday
NARUTO
true
Tanmay
thanks so much my friend
Youssef
Dear tanmay and other friends I want to ask that if both sun and earth are moving away from their center than why their inter relative distance is increasing rather squeezing taking in consideration of masses I am student so I apologize further and wanna guidance if possible
haseeb
The Earth does not get close to or “touch” the Sun because it is in orbit around the Sun. A circular orbit means that the orbiting body always stays the same distance from the thing it orbits. The Earth's orbit is nearly circular and keeps us about 93 million miles from it.
Konica
Konica nor the orbit can prevent our earth from getting closer (if it was coming closer) and nor is the orbit of earth nearly circular
Tanmay
And Haseeb can you make your question a little simple or something I didn't get it. What do you mean by moving away from their center?
Tanmay
And then comes your increasing rather than squeezing taking mass into consideration I never heard of that and I didn't understand that
Tanmay
Which formula are you using for that squeezing part?
Tanmay
What's difference between Astronomy and Astrophysics?
Gurjinder Reply
Actually astrophysics is one of the many branches of astronomy. It usually deals with the mathematical calculations of predicting the motions of heavenly bodies and also it studies the evolution of our universe by using physics.
Debabrata
So is there any observational astrophysics?
Gurjinder
Yes there are 2 types of astrophysicsist one is theoretical and other one is observational. I don't know about astrophysics though
Tanmay
Thanks
Gurjinder
welcome
Tanmay
😊
Gurjinder
Is there a way for us to find out that we are in a simulation or not?
Tanmay Reply
I think mathematics is a key
Arghyabindu
If we are in a simulation then I don't think that the super intelligent species which is controlling this simulation will give us any hint about how to know it.
Debabrata
There has to be some kind of bugs even a carefully designed app has some bugs in our world
Tanmay
Yeah.... There might be some loophole in their programme code.... And who knows.... We could find the bugs in a super massive black hole
Debabrata
But here we are talking about a very very very massive simulation by a very advanced species...... So they might ve already planned on how to deal with all the bugs and glitches
Debabrata
There's a really interesting theory that states even if we are in a simulation, it would be coded in such a way that as soon as one of us (a sentient being inside the simulation) had the slightest hint/evidence that we were in a simulation, the simulation would be edited to remove that information
ben
(I keep commenting on posts and they just dont post sometimes, does anyone else have this problem or should I just delete the app?)
ben
same happened to me but I see yours))
NARUTO
(ah well it must just be a little problem with the app, cheers for letting me know 🙌)
ben
(Well never happened to me though it takes some time to post large messages)
Tanmay
I saw a comment where he told that the universe is a game and finding it's nature is the level as soon as we find its nature the universe collapse and make another universe with more complex nature and this continues
Tanmay
(ahh that may be what it is actually because it is only the longer messages, thanks for letting us know)
ben
That was really a unique way of seeing the universe
Tanmay
Impractical way are there such as leaving your mortal body and entering to the astral body....cons of this idea is you can't tell us what you have saw or feel in that endemic confrontation
Arghyabindu
what a friend told me now
Arghyabindu
We are just a component of the universe trying to understand the other components of the universe...... So I guess that sometimes it makes the matter more and more complicated
Debabrata
what a friend told me now
Arghyabindu
Wait if we are in a simulation then does that means our future is predicted and we can't change it like some kind of rpg game
Tanmay
Yes IF we are in a simulation then definitely the future is already programmed..... But it may be in early access stage..... And the future might not be written at the moment.
Debabrata
so we can do time travel upto the point where our future is written
Tanmay
Maybe or may not be..... Depending on the script or program written in the simulation..... The intelligent species should be controlling every step we take and every thought that come into our mind.
Debabrata
Have you seen sword art online anime?
Tanmay
well never mind it's just repeated stuff
Tanmay
Nah.... I don't watch anime
Debabrata
ok
Tanmay
hii guyz
Lambent
Hey
Debabrata
BLM ✊🏾✊🏾✊🏾
Laila
?
Kilten
thanks
Sahil
If we can simulate consciousness somehow, then there is a bright chance that we 'might' be living in a simulation all along.
Ganesh
which is the biggest star in the universe? if anyone know answer,please reply.
Ciffee Reply
which is the biggest star. ?
Ciffee
u y scuti
Sunidhi
sun
Konica
for our galaxy
Konica
Konica UY Scuti is the biggest Star ever observed and it's located in our galaxy.
Gurjinder
what are the branches of astronomy?
SCIENCE Reply
So there are literary many branches of astronomy like- Radio astronomy, Ultraviolet astronomy, X ray astronomy, etc. (just imagine the electromagnetic spectrum) There's also Astrophysics (My favorite), astro chemistry, astrobiology, astro geology, planetary science, astrometry
Debabrata
factors that contribute to the development of ancient astronomy to modern astronomy
Andrea Reply
When will be we found them?I am so waiting.
Everything Changes when Galileo Galilee observed Sky with a telescope. There's no doubt that ancient astronomers knows a lot of things about sky , but there observation was with naked eye. i wanna share some of the glimpse of Indian ancient astronomy:
Gurjinder
1.They know the difference between the Star and Planets at that time. 2. They know which star is multiple star system and their rotation with respect to each other. 3. They know the value of speed of light. And their are lots of things which they knows even without telescope.
Gurjinder
why is a supernova
pooja Reply
well there are different types of supernovas. there is a type 1a supernova and that happens when a white dwarf reaches the mass of 1.4 of the sun. A type 2 supernova is when a high mass star starts to fuse iron in the core and the star collapse then expands to create that supernova
PopularHellboy
what is time.!
Samyak Reply
92 billion light years
The
4th Dimension
The
time is illusion
Ciffee
Time is a way of describing an event happened in our 3D world
Tanmay
Why are the largest visible-light telescopes in the world made with mirrors rather than lenses?
Nibha Reply
What is a charge-coupled device (CCD), and how is it used in astronomy?
Nibha
Why is it difficult to observe at infrared wavelengths? What do astronomers do to address this difficulty?
Nibha
not sure what a CCD.
PopularHellboy
(also, lenses have are more problematic over larger distances due to the way different wavelengths of light act within glass. Red and blue light wavelength differs by about 300nm, and so the refraction will affect them differently, whereas when reflecting on a mirror, this is not a problem)
ben
hi
Zalka
i'm from Indonesian
Zalka
(I tried posting this before but this app doesn't seem to work properly) A CCD essentially works as a camera. It's made from an array of capacitors (which make up the 'pixels'). When an incident photon hits the 'pixel', the charge generated is measured and can be translated into an image we can view
ben
the reason mirrors are more common than lenses (on large scale telescopes) is because it is a lot easier and cheaper to make a parabolic mirror than it is to make a perfect glass lens
ben
Because Refracting Telescope cause Cromatic abberetion.
Tushar
Just reading along. I can't see the end of the page so commenting to move it up.
Eva
same. but mirrors are also made with great efficiency and calculation for trapping the light
Konica
How might Venus’ atmosphere have evolved to its present state through a runaway greenhouse effect?
Nibha Reply
what is synchrotron radiation
Nibha
I am 15 and I got interested in astronomy. Can anyone tell me the basics that I must know?
Vikky Reply
you can start by studying the theories of astronomy. there is light and the spectrum of electromagnetic radiation. from there you learn about waves and how we as astronomers received information from space. then you can learn about the nebula hypothesis and also about our solar system
PopularHellboy
but if you learn about the foundation of light and the dually of nature with it and the Doppler effect, it will apply to most stuff in astronomy
PopularHellboy
heyy @populadHellboy can you tell me from where can I find this Light and The spectrum of electromagnetic radiation ? I mean is there any course ?
Sagar
I mean I learn astronomy in college with astronomy 101 and 102 I'm sure if you search online for YouTube videos or blogs they can help you with that. try crash course on YouTube for an introduction
PopularHellboy
Currently preparing for olympiad, is there any online resources for Astronomy, astrophysics, and cosmology? If there's any past question papers i'll be delighted. I'm currently using Principle and Practice and Introduction To Modern Astrophysics..
Aditya
what is time
Sourav
how big is a parsec
David Reply
I believe it's about 3.086 x 10^16 meters. You can try converting 1 radian to 1 arc secs, multiply with 1 AU, convert it to meter.
Aditya
Its 31 trillion km
Asim
very big
Ciffee
Practice Key Terms 6

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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