Could we see a galaxy that is 20 billion light-years away?

No. It's beyond the bounds of our observable universe. The universe is only about 15 billion years old. Their light wouldn't have time to reach us.

Light year is the distance in which light travels in an year. For example, if we see a celestial body which is a billion light years away, that means light has taken one billion years to reach earth. Our universe is around 13.7 billion years old, thus we can see the objects which are not more than 13.7 billion light years away. This is because light has not reached the time yet to come to the earth.

Summarizing, we cant see object 20 billion light years away.

A lot of interesting answers. However, this question can be considered in two mutually exclusive ways:

First, from the standpoint of what we know of physics and cosmology at present, then our observable universe , as defined by several answerers, is not as large as 20 billion light years in radius and nothing at that distance, if it existed at all, would be detectable at all.from the earth.

On the other hand, the "observable universe" is really just an abstract ides, based upon the measured red shifts of distant celestial bodies and Einstein's special and general theories of relativity which invoke "c" as the ultimate cosmic speed limit.

Between these two opposing views there is really no reason to choose. It is possible to hypothesize a cosmos in which "c" is not the final limit of velocity.The concept of wormholes postulates a cosmos in which infinite velocities are routine. In addition,string theory., if true,also involves a limitless cosmos. In fact, even without these exotic ideas, quantum mechanics itself (which Einstein never accepted because it negated his idea of a knowable and predictable universe) which is generally felt today to be a "truer" description of reality than either Newtonian or Einsteinian Physics,

Finally, there is the viewpoint which is common to all systems of human belief and which postulates the existence of a Creator who/which can design the universe any damn way he/it chooses. There is no more evidence for any one of these postions rhan another, so it really all comes down to a matter of psychology, faith, and rerlatively feeble attempts to rationalize the cosmos according to one set or another of arbitrary laws.
All that being said, your teacher is certainly looking for the second answer, so unless you want to show him/her up as a boob, you"re probably wiser to stick to that response.

As far as your "additional details" question goes, I'd say that it depends on which of the alternatives above you subscribe to. For me, the answer is "Mayhaps so, mayhaps not."

The universe has been around for 15 billion years so the light has only 3/4 of the way to earth. Which means its nearly 47 302 642 000 000 000 000 000 kilometers away from us.

If you cant wait that long well you could travel in a spaceship that travels at the speed of light, wait for 2.5 billion years and you will see the galaxy.

Theoretically yes, but about 6 billion years from now.

Light can only travel so fast, and since the universe is only about 14 billion years old, the light would take that much longer to reach us.

We can only see galaxies whose light has had time to reach our telescopes. If you see a star that is 4 light years away, then you are seeing it was it was 4 years ago. The answer is "No, because it would be beyond the bounds of our observable universe." I believe that the farthest we have seen is 15 billion light years or so, and it is currently impossible to prove the size of the universe / what can or cannot possibly exist.

The question is ambiguous---in cosmology, for large distances, there are several different possible meanings to the phrase "a galaxy that is 20 billion lightyears away", and it's important to distinguish between them.

If she means "Is it possible to see a galaxy whose angular diameter distance is 20 billion lightyears?" the answer is no, in fact there are no galaxies whose angular diameter distance is greater than 4 billion lightyears.

For various other measures of distance, such as luminosity distance, the answer is also "no".

If she means "Is it possible to see a galaxy whose Lemaitre model radial coordinate is greater than 20 billion lightyears?" the answer is yes, because although our apparent event horizon is at a radius of 13.7 billion lightyears, the material that appears to us just within our event horizon has (since those photons where emitted) expanded with the Hubble flow. So we can see galaxies that are "now" 20 billion lightyears away, although to us they will appear to be only about 11 billion lightyears away.

When the Universe is 20 billion years old, 6.3 billion years from now, our event horizon will be at an apparent distance of 20 billion lightyears.

An international team of astronomers may have set a new record in discovering what is the most distant known galaxy in the Universe. Located an estimated 13 billion light-years away, the object is being viewed at a time only 750 million years after the big bang, when the Universe was barely 5 percent of its current age. The primeval galaxy was identified by combining the power of the NASA/ESA Hubble Space Telescope and CARA's W. M. Keck Telescopes on Mauna Kea in Hawaii. These great observatories got a boost from the added magnification of a natural ‘cosmic gravitational lens’ in space that further amplifies the brightness of the distant object. The newly discovered galaxy is likely to be a young galaxy shining during the end of the so-called "Dark Ages" - the period in cosmic history which ended with the first galaxies and quasars transforming opaque, molecular hydrogen into the transparent, ionized Universe we see today.

The new galaxy was detected in a long exposure of the nearby cluster of galaxies Abell 2218, taken with the Advanced Camera for Surveys on board the Hubble Space Telescope. This cluster is so massive that the light of distant objects passing through the cluster actually bends and is amplified, much as a magnifying glass bends and magnifies objects seen through it. Such natural gravitational ‘telescopes’ allow astronomers to see extremely distant and faint objects that could otherwise not be seen. The extremely faint galaxy is so far away its visible light has been stretched into infrared wavelengths, making the observations particularly difficult. "As we were searching for distant galaxies magnified by Abell 2218, we detected a pair of strikingly similar images whose arrangement and colour indicate a very distant object," said astronomer Jean-Paul Kneib (Observatoire Midi-Pyrénées and California Institute of Technology), who is lead author reporting the discovery in a forthcoming article in the Astrophysical Journal. Analysis of a sequence of Hubble images indicate the object lies between a redshift of 6.6 and 7.1, making it the most distant source currently known. However, long exposures in the optical and infrared taken with spectrographs on the 10-meter Keck telescopes suggests that the object has a redshift towards the upper end of this range, around redshift 7.

Redshift is a measure of how much the wavelengths of light are shifted to longer wavelengths. The greater the shift in wavelength toward the redder regions of the spectrum, the more distant the object is.

"The galaxy we have discovered is extremely faint, and verifying its distance has been an extraordinarily challenging adventure," said Dr. Kneib. "Without the 25 x magnification afforded by the foreground cluster, this early object could simply not have been identified or studied in any detail at all with the present telescopes available. Even with aid of the cosmic lens, the discovery has only been possible by pushing our current observatories to the limits of their capabilities!"

Using the combination of the high resolution of Hubble and the large magnification of the cosmic lens, the astronomers estimate that this object, although very small - only 2,000 light-years across - is forming stars extremely actively. However, two intriguing properties of the new source are the apparent lack of the typically bright hydrogen emission line and its intense ultraviolet light which is much stronger than that seen in star-forming galaxies closer by.

"The properties of this distant source are very exciting because, if verified by further study, they could represent the hallmark of a truly young stellar system that ended the Dark Ages" added Dr. Richard Ellis, Steele Professor of Astronomy at Caltech, and a co-author in the article.

The team is encouraged by the success of their technique and plans to continue the search for more examples by looking through other cosmic lenses in the sky. Hubble's exceptional resolution makes it ideally suited for such searches.

"Estimating the abundance and characteristic properties of sources at early times is particularly important in understanding how the Universe reionized itself, thus ending the Dark Ages," said Mike Santos, a former Caltech graduate student, now a postdoctoral researcher at the Institute of Astronomy, Cambridge, UK. "The cosmic lens has given us a first glimpse into this important epoch. We are now eager to learn more by finding further examples, although it will no doubt be challenging."

"We are looking at the first evidence of our ancestors on the evolutionary tree of the entire Universe," said Dr. Frederic Chaffee, director of the W. M. Keck Observatory, home to the twin 10-meter Keck telescopes that confirmed the discovery. "Telescopes are virtual time machines, allowing our astronomers to look back to the early history of the cosmos, and these marvellous observations are of the earliest time yet." Notes for editors The team reporting on th