Normally, objects that are further away look smaller. That is the basic rule of perspective. Without this rule, everything would be infinitely large right in front of us and we wouldn't be able to see anything. So, thank you, perspective. In outer space, however, if we look deep enough, objects start to look larger than they should, as if they are closer. This is because the universe has been expanding, and early objects are seen at a time when the universe was much closer together. This phenomenon is rarely mentioned by astronomers in public forums and then only in passing, without much excitement.
Maybe because I am a visual artist, I can't quite get over this notion. And, because I am a complete amateur when it comes to Astronomy, I am perhaps misunderstanding the situation, so, please correct me if this is the case. As far as I can tell however, from researching the published science, this is actually the case, or believed to be the case. I haven't come across any actual observations. Also, it can only be the case if the big bang theory turns out to be true. And, vice-versa, if the very distant objects do not look enlarged (yet fainter) the theories of the big bang and the expanding universe are challenged.
I came upon this notion when the first Hubble Deep Field images were published and raised in me some questions regarding the optics of the expanding universe. I call it the Deep Field paradox.
In 1995 NASA created an image called
the Hubble Deep Field (HDF). It was made using hundreds of long
exposures at different wavelengths. It shows a region of the sky the
size of a grain of sand held at arm's length (see more analogues and
superlatives at the end of this post) in the greatest detail ever,
exposing extremely far away galaxies, at a very early time in the
history of the universe. It is considered by many the greatest
photograph ever taken. But, more importantly, the image was the
first to show a density of galaxies in what was until then assumed to
be an empty region of the sky, indicating that there are far more
galaxies in the visible universe than we had known before. The patch
of sky was selected to avoid interference from other closer objects.
Several more images have been created since then, such as the Hubble
Ultra Deep Field (HUDF), the (Extremely Deep Field Image (XDF), of
the same regions, and with even greater enlargement, showing objects
even further back in time. The Hubble Ultra Deep Field South focused on a similarly blank region in the southern hemisphere and
showed that this density of galaxies is probably uniform in all directions.
Some of the objects in the image are
seen at only a few hundred million years after the big bang, which is
now believed to be close to 13.8 billion years ago. This ever
increasing reach towards the temporal beginning of the universe, implies that
we will eventually be able to see the big bang itself, because our
technology keeps improving. While it is true that we will still see
further back than at present, there is actually a limit, because
there was a period, referred to as the dark ages, during which nothing was visible, where a kind of
murky fog persisted. Once our telescopes reach that far back,
visibility will stop (although we can already detect cosmic background radiation, and kind of indirect image of the big bang, or maybe not).
But the image still raised two
questions for me, one of which turned out to be based on a
misunderstanding, the other turned out to be a very strange paradox,
which am still trying to fully understand.
The first question that I had regarding
this image and the notion of the expanding universe is a common one
and is often phrased as: “How can we see the early universe and the
Big Bang? Shouldn’t the light have already passed us?” (By “us”
I mean not us presently but the stuff that we are made of, our
previous materials in the past.) If everything was so close together
and nothing can expand faster than the speed of light shouldn't all
that light and information have already gotten to us once before and
then how can we now see it again?
This questioning logically follows several misunderstandings based on the simplifications and analogies of astronomers trying to explain the universe to us laymen. The explanation begins like this: The visible universe started out as a singularity, a point so small it was without dimension and then very rapidly expanded into a ball the size of (insert here golf ball, grapefruit, orange, etc) and into space many light years across. It is still expanding, though less rapidly. This narrative is factually correct, as far as the scientific consensus is concerned, but the language creates a lot of confusion. When the golf ball is mentioned (or some fruit) astronomers are talking about the very early size of the CURRENTLY VISIBLE universe and NOT (and this is key) the ENTIRE universe. By visible they mean observable, as in limited by time. (Sometimes, by observable they mean opposed to invisible to our eyes, as in infrared, or because it is dark energy or dark matter, but not here.) So imagine a kind of large “sphere of the visible” 13.7 billion years in temporal diameter in all directions, and shrink it down to a golf ball. The edges of the ball represent “then” what the outer visible edges are now.
But that sphere is not all there is. The universe goes on from there. And the universe “out
there” on “the edge” is not any older or denser than around
here, only in the image of it. So if we waited another billion years,
the observable universe would be a billion light years “larger”
or "further out," showing us even more. The same would happen if we could
instantly transport to a place a billion light years away and looked
out in the same direction that we travelled, we would see another
billion light years further. So, the universe continues beyond the
HUDF image, we just can't see it yet. This photograph, like any others
that have gone and will go deeper, does not show “the edge”
of the Universe, just a period close to the beginning of time (which
is amazing enough).
How much more of the Universe is there?
That is still being debated. It could be infinite, also known as
flat, or finite and curved (like a saddle or a sphere but in higher
dimensions). In either case it is much larger than what we can now
observe.
Let's get back to the golf ball. At that
moment in early expansion, there would be many more golf balls,
possibly an infinite number of them, so it makes sense that we didn't
already see all of the universe at that moment in time.
But this infinity still doesn't explain
why we wouldn't have seen at least the inner edges of our golf ball at
that time. The explanation for this requires the mentioning of the
second misunderstanding regarding space time, which is the speed of
light limitation. Early on (and still today) the universe expanded at
much faster than the speed of light. But isn't this impossible? No.
Light within space cannot travel faster than the speed of light, but
space itself can. Space expands equally everywhere very slowly, but
distances accumulate over distance, so the farthest away objects are
receding much faster (relative to us) than the speed of light. The outer rim of the
golf ball never had a chance to optically reach us THEN because space
was expanding so much faster than light. It has reached us now in the
form of those earliest galaxies. Why now? Because, by definition what
reaches us now is the visible edge. What reaches us tomorrow is the –
slightly larger – visible edge tomorrow. The answer to “Shouldn’t
the light have already passed us?” is that space expansion was
faster than light and yet the light eventually reaches us. For the objects whose light age is close to the
age of the universe, it took that long to reach us. Imagine someone
in a car that is driving away from you, and throws a ball towards you.
Even if the car was very close to you at first, the ball would take a
while longer to reach you because of the movement of the car.
These deep field objects are very far away because they are extremely
redshifted. Redshift, the reddening of light waves, or expanding of
all electromagnetic waves, is due to a distortion of space and its
effect on light across space-time. Redshift, discovered by Edwin
Hubble, is extremely important as a measure of the age of light, and
therefore distance. But, the age of light (light years) as a measure
of distance is a misleading concept in deep space. Light years
indicate the distance travelled through space, which gets distorted
at larger distances, and so no longer matches actual
distances in time.
A typical quote that promotes the
misunderstanding regarding time versus distance is from Time magazine
online: “The XDF [Extreme Deep Field image] goes even farther [than
the HDF image], capturing objects some 13.2 billion light-years
away”.
It doesn't. It captures light that is 13.2 billion light years old,
which in turn tells us a lot about its distance, but that IS NOT its
distance. Objects whose light is 13.2 billion light years old are
32.69 billion light years away NOW, because after they released their
light, they moved away from us in the expansion of space.
With close-by objects the time
distortion is a comfortable concept. We see the moon 1.5 seconds in
the past. We see the sun 8 minutes ago. Mars, 20 minutes ago, the
nearest other star, Proxima Centauri 4.2 years ago. But we see these
objects at roughly their correct distance, according to conventional
perspective and the relationship between the speed of light and
distance. However, when we observe other galaxies, the great
distances mean that we see things not only in the past, but at
drastically wrong distances, because the subtle expansion of space
accumulates exponentially and becomes a strong distortion.
This distance distortion, the fact that
there is a discrepancy between the age of the light (in this case
13.2 billion years) and their present position (32.69 billion years
away) is also not a difficult concept. Light takes time to travel so
things are not where they used to be (and look very differently now).
But there is a third distortion, and
this is the Deep Field paradox: some of the furthest away objects
appear closer to us, and larger, than some of the closer objects.
This violates the rules of basic perspective (farther appears
smaller). It's a kind of telescoping near the edge of the visible. I
suspected this early on while looking at the image, and this was my second, totally naive yet reasonable question: shouldn't objects from the
earliest periods of space appear very close to us? Apparently they
do.
I came across the first confirmation of
my suspicion in this quote:
“In an expanding universe, we see the
galaxies near the edge of the visible universe when they were very
young nearly 14 billion years ago because it has taken the light
nearly 14 billion years to reach us. However, the galaxies were not
only young but they were also at that time much closer to us. The
faintest galaxies visible with the Hubble Space Telescope were only a
few billion light years from us when they emitted their light. This
means that very distant galaxies look much larger than you would
normally expect as if they were only about 2 or 3 billion light years
from us (although they are also very very faint - see Luminosity
Distance) [emphasis added].” http://www.atlasoftheuniverse.com/redshift.html
The same object mentioned before, 13.2
billion light years in the past, was only 2.9861 billion light years
away when it released that light, because back then space was much
more condensed. Its image appears to us in a position much closer
than its age in years would indicate, about 1/5th the
distance. Its original distance to us i.e. its apparent size when the light left, is defined by astronomers in
terms of its “angular size”. I think of it as “how close it
appears”.
You can play around with astronomical
distances at this great website, the Light Travel Time Converter: http://www.astro.ucla.edu/%7Ewright/DlttCalc.html Just enter a number for light travel
time in Gyr (billion years), click on flat (infinite universe), and
the rest of the measurements appear. Its apparent distance, will be listed as angular size distance. Try any
number between 0 and 13.72 (the age of the universe). Then try
13.7199 and .046. Notice that an object whose light is 13.7199
billion light years (Gly) light travel distance would
appear as if it is only .0.045587
Gly (about 45.587 million light years), which is closer than
than an object whose light is only .046 light years “away”, at
0.045924
Gly.
So an object that is cosmologically
very far away has a larger angular size than an equivalent object that is much
closer. It appears closer, not because it IS, but because it WAS
(when its light left.) The furthest objects gave off their light when
closer to us, earlier on, when space was much more condensed, and so
appear larger than others that are closer to us, but whose light left
more recently. In theory, if we could actually see the foggy, murky
space right after the big bang, the beginning of time itself, it
should appear right in front of us. Maybe it does.
When you add a value of 13.72 Gly, the time of the big bang, in the Light Travel Time Converter, you get a distance of 0.
When you add a value of 13.72 Gly, the time of the big bang, in the Light Travel Time Converter, you get a distance of 0.
To illustrate the notion of enlarged distant objects, I made
an image that shows an object as it would appear in a deep field
image. Because galaxies not only change drastically in shape, but
also in size and color over time, they do not make for the perfect
familiar object in a demonstration. So instead I use an actor's face,
someone famous and pictured for his entire life: Mickey Rooney (I
could have just as well used Shirley Temple or Michael Jackson). Like
galaxies, people look younger in older images. Each row represents
copies of him from left to right at different cosmological distances,
vertically increasing with successive distortions.
Row A: Five identical Mickey Rooneys
Row B: Five Mickey Rooneys each further
back in time/space
Row C: With added perspective (they
look smaller with distance)
Row D: With added red shift (they look
redder with distance)
Row E: With added dimming (they look
fainter with distance)
Row F: The Deep Field paradox (the
farthest look much larger than they should)
And here's a simulation of a deep field image with many Mickey Rooneys at different distances.
Notes:
The big question responded to on askamathematician.
"The
farther we see into the universe, the younger the objects we see. But
if we could see the Big Bang, it would be right on top of us. Isn't
this paradoxical?"
A list of the superlatives and oddities
of the Hubble Ultra Deep Field.
• it took one week of imaging, so it
is a very long exposure. This long exposure was necessary to collect
enough light to see resolve the faintest galaxies.
• it shows the furthest away and
oldest objects in the universe every captured (up to that time).
• it shows the universe as it was
between 400 and 800 million years after the big bang, about 12.9 to 13.3
billion years ago. The current consensus for the age of the universe
is 13.7 billion years.
• it looks like a picture of stars,
but on closer inspection it is almost exclusively galaxies, over
10,000.
• the section of the sky pictured is:
if you made 8 feet of drinking straws into one tube and looked
through it at the sky, or 1/10 of the width of the full moon, or a
the size of a tennis ball seen from 300 feet away, or a grain of sand
held at arms length, or 24-millionth
of the whole sky.
• this is not a crowded section or
unusual section of the sky, it is dense with with galaxies like this
in every direction
• many objects are warm in color; the
redder they are, the older and further away they are, due to redshift. The objects are in actuality in the infrared spectrum and we can't see them without using
infrared sensors.
• the farthest imaged objects are
young galaxies, therefore smaller and less ordered.
• many galaxies shown are larger,
brighter, or strangely distorted (see “face galaxy”
http://msp.warwick.ac.uk/~cpr/paradigm/HUDF-2.pdf) than they should
be because of gravitational lensing. This lensing, which acts like naturally
occurring gravity-based telescopes, allows astronomers to see even
“deeper” into space, by providing naturally occurring
gravity-based telescopes. Because these “lenses” are irregular in
shape, they can strangely distort and multiply objects. Multiple
versions of the same objects can also be seen at different periods in
their history.
• all optical rules that apply to this image also apply to any other image, even with much closer objects, just less so.
• all optical rules that apply to this image also apply to any other image, even with much closer objects, just less so.
