Why is space dark?
Tom Hix
billions and billions of stars in the universe, the sky at night between
the stars is still jet black. Of course I know nothing at all about the
universe, but it would seem to me with all those stars out there
emitting light, that the universe would be very bright instead of dark.
Is it because the universe is so large and distances are so great that
the light becomes so diffused and scattered, that by the time it
reaches us, only faint glimmers of light can be seen? I know this is a
dumb question but it is something that has always bugged me.
Thanks for your responses
Tom Hix
thre...@webtv.net
stars. No one has given proof that the universe is indeed infinite. It
might be. It might not be. Hundreds of billions of galaxies times
hundreds of billions of stars in each galaxy gives an incredibly large
number, but not infinite :-)
We respectfully request that you take us to Eden -- Doctor Sevrin :-)
WirtAtmar
>Will someone please explain to me in (laymans terms) why with all the
>billions and billions of stars in the universe, the sky at night between
>the stars is still jet black. ...
>I know this is a
>dumb question but it is something that has always bugged me.
Actually, this is a fairly profound question in astronomy -- and it even has a
name: Olber's Paradox. If you'd been born two hundred years earlier, it could
well have been called Hix's Paradox.
Being bugged by a problem and diligently seeking out an honest answer is all
there really is to science. As to the currently accepted answer to yours and
Olber's question, please see:
Wirt Atmar
n2_space
that way...that is the easiest way to think of it...
Jeremy Bingham
President - The Louisville Astronomical Society
remove star before "@" to send e-mail
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J. Scott Miller
>
> Will someone please explain to me in (laymans terms) why with all the
> billions and billions of stars in the universe, the sky at night between
> universe, but it would seem to me with all those stars out there
> emitting light, that the universe would be very bright instead of dark.
> Is it because the universe is so large and distances are so great that
> the light becomes so diffused and scattered, that by the time it
> dumb question but it is something that has always bugged me.
> Tom Hix
The short answer is two-fold: 1) The expansion of the universe redshifts
light coming from objects at greater and greater distances from us, moving it
into wavelength bands we cannot see, and 2) the universe is finite in age, so
that there has been insufficient time for light to travel to us for objects at
distances greater than about 12 to 13 billion lightyears. The latter is more
to the point than the former.
More formally, from Ned Wright's Cosmology FAQ, linked from
http://www.astro.ucla.edu/~wright/cosmolog.htm
Why is the sky dark at night?
If the Universe were infinitely old, and infinite in extent, and stars could
shine forever, then every direction you looked would eventually end on the
surface of a star, and the whole sky would be as bright as the surface of the
Sun. This is known as Olbers' Paradox after Heinrich Wilhelm Olbers
[1757-1840] who wrote about it in 1823-1826 but it was also discussed earlier.
Absorption by interstellar dust does not circumvent this paradox, since dust
reradiates whatever radiation it absorbs within a few minutes, which is much
less than the age of the Universe. However, the Universe is not infinitely
old, and the expansion of the Universe reduces the accumulated energy radiated
by distant stars. Either one of these effects acting alone would solve Olbers'
Paradox, but they both act at once.
--
J. Scott Miller, Program Coordinator Scott....@louisville.edu
Gheens Science Center and Rauch Planetarium
http://www.louisville.edu/planetarium
University of Louisville
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Parallax
wrote:
>
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>
>In the part of space that we are in is black, but what is to say that
>somewhere else in space it is all light?
It is. I can name many places where it's all light.
Inside the Sun.
Inside Sirius.
Inside ....
--Parallax
rosa nina williams
>>
>> Will someone please explain to me in (laymans terms) why with all the
>> billions and billions of stars in the universe, the sky at night between
>> the stars is still jet black.
"J. Scott Miller" <Scott....@louisville.edu> writes:
>The short answer is two-fold: 1) The expansion of the universe redshifts
>light coming from objects at greater and greater distances from us, moving it
>into wavelength bands we cannot see, and 2) the universe is finite in age, so
>that there has been insufficient time for light to travel to us for objects at
>distances greater than about 12 to 13 billion lightyears. The latter is more
>to the point than the former.
And me, I add,
As an observer, I have to put in a word for (3) clouds of interstellar
gas and dust intervene along our lines of sight, so that at some point,
light from distant stars is absorbed or redirected by this material.
(Of course, that material may well re-emit light of its own, but that
is often in other wavelength bands, invisible to the eye - see (1) above.)
Without all this interstellar equivalent of what you find under your couch,
we'd see a great deal more light. Not an *infinite* amount, as per (1)
and (2) above, but more than we have now. This is particularly the case
for our own Galaxy - take a look at your favorite picture of the Milky
Way, and note the big dark streaks. Those are caused by said interstellar
crud, blocking out the light from the lots and lots and lots of stars that
are right in our own neighborhood (relative to the expanse of the entire
universe, anyhow). If you hear someone kvetching about "obscuration,"
that's what they're upset about.
--Rosa Williams
Bob May
threshold to overcome to be able to see any light. The eye seems to have a
"exposure time" of about 1/15th of a second or so anything that doesn't trip
the eye's sensors in that time period aren't seen and if it happens
occasionally, then the brain considers it noise in the system and ignores
it. You also see a star and your eye adjusts to that as the white and thus
the darker area around the star ends up being black due to the relative lack
of light. It's all relative in a way.
Have Fun and Keep Looking Up.
Bob May
scott torvic
Gavin Tabor
>
> Tom Hix wrote:
> >
> > Will someone please explain to me in (laymans terms) why with all the
> > billions and billions of stars in the universe, the sky at night between
> > universe, but it would seem to me with all those stars out there
> > emitting light, that the universe would be very bright instead of dark.
> > Is it because the universe is so large and distances are so great that
> > the light becomes so diffused and scattered, that by the time it
> > reaches us, only faint glimmers of light can be seen? I know this is a
> > dumb question but it is something that has always bugged me.
> > Thanks for your responses
> > Tom Hix
>
> light coming from objects at greater and greater distances from us, moving it
> into wavelength bands we cannot see, and 2) the universe is finite in age, so
> that there has been insufficient time for light to travel to us for objects at
> distances greater than about 12 to 13 billion lightyears. The latter is more
> to the point than the former.
1. is irrelevant. The redshift also shifts UV, X and gamma wavelengths
to visible light. 2. is the true explanation.
Gavin
>snip
> --
> J. Scott Miller, Program Coordinator Scott....@louisville.edu
> Gheens Science Center and Rauch Planetarium
> http://www.louisville.edu/planetarium
> University of Louisville
--
Dr.Gavin Tabor
email : ga...@ic.ac.uk
home page : monet.me.ic.ac.uk/people/gavin/gavin.html
Department of Mechanical Engineering,
Imperial College,
London SW7 2BY
J. Scott Miller
>
> J. Scott Miller wrote:
> > The short answer is two-fold: 1) The expansion of the universe
> > redshifts light coming from objects at greater and greater distances
> > from us, moving it into wavelength bands we cannot see, and 2) the
> > universe is finite in age, so that there has been insufficient time
> > for light to travel to us for objects at distances greater than
> > about 12 to 13 billion lightyears. The latter is more to the point
> > than the former.
>
> 1. is irrelevant. The redshift also shifts UV, X and gamma wavelengths
> to visible light. 2. is the true explanation.
>
> Gavin
Then let me elaborate. 1. is relevant because the expansion of the universe,
and the redshift that occurs, causes a diminishing of the energy of the
photons emitted from the surface of the object, regardless of the energy at
which it was emitted. Calculations show that this effect alone can account
for a dimming of the background by a factor of two.
Thus, it is not the sole solution, but contributes to the whole.
(reference: Jay Pasachoff, "Astronomy: From the Earth to the Universe")
Nice
<375E6A72...@louisville.edu>...
blueshift of the same magnitude, the night sky would still be dark. An
entire book has been written about Olbers' paradox and discusses wrong
solutions (redshift, interstellar absorption, inability to perceive faint
light, etc.) as well as the two correct solutions (finite age of the
universe, lack of energy in the universe): DARKNESS AT NIGHT by Edward
Harrison.
Do NOT trust astronomy textbooks. A 1987 study (Wesson, Valle, and
Stabell; ApJ 317, 601) found that 7 out of 10 astronomy textbooks
explained Olbers' paradox incorrectly: "Of 10 books, only three have a
discussion that may loosely be termed right (Berry 1976; Harrison 1981;
Kaufmann 1985). Another five are either confused or ambiguous (Landsberg
and Evans 1977; Silk 1980; Raine 1981; Friedlander 1985; Hartmann 1985).
And two can only be termed wrong (Mitton 1977; Zeilik 1982)."
Historically, the reason that many people still believe the redshift
explains the dark night sky is because Olbers' paradox was rediscovered in
the 1950s--by steady state cosmologists. In the steady state cosmology,
the redshift DOES explain the dark night sky. But it is NOT the solution
in the big bang cosmology.
J. Scott Miller
> The redshift IS irrelevant. If you converted every galaxy redshift into a
> blueshift of the same magnitude, the night sky would still be dark. An
> entire book has been written about Olbers' paradox and discusses wrong
> solutions (redshift, interstellar absorption, inability to perceive faint
> light, etc.) as well as the two correct solutions (finite age of the
> universe, lack of energy in the universe): DARKNESS AT NIGHT by Edward
> Harrison.
So, just for arguement sake, this reference is to be believed while that of
other astronomers is not.
>
> Do NOT trust astronomy textbooks. A 1987 study (Wesson, Valle, and
> Stabell; ApJ 317, 601) found that 7 out of 10 astronomy textbooks
> explained Olbers' paradox incorrectly: "Of 10 books, only three have a
> discussion that may loosely be termed right (Berry 1976; Harrison 1981;
> Kaufmann 1985). Another five are either confused or ambiguous (Landsberg
> and Evans 1977; Silk 1980; Raine 1981; Friedlander 1985; Hartmann 1985).
> And two can only be termed wrong (Mitton 1977; Zeilik 1982)."
But, who is to say whether the text I have is as dated as the listing above.
Though I will agree that they all don't get it right, comparisons of older
texts with newer versions have demonstrated corrections as needed. And,
indeed, I quote from my current version of Kaufmann (Universe, 5th edition):
Besides the finite age of the universe, a second effect also contributes
significantly to the darkness of the night sky - the redshift. According to
the Hubble law, the greater the distance to a galaxy, the greater the
redshift. When a photon is redshifted, its wavelength becomes longer and its
energy - which is inversely proportional to its wavelength - decreases.
Consequently, even though there are many galaxies far from the Earth, they
have large redshifts and their light does not carry much energy. A galaxy
nearly at the cosmic particle horizon has a nearly infinite redshift, meaniing
that the light we receive from that galaxy carries practically no energy at
all. This decrease in photon energy because of the expansion of the universe
decreases the brilliance of remote galaxies, helping make the night sky dark.
[end quote]
So, by your listing above, Kaufmann had it substantially right in his 1985
version of his text (I believe I have that copy at home to check with) and it
is not likely that he changed the above significantly in this current
edition. Maybe he did, and I will mention it in a future post if I do have
the text in question.
>
> Historically, the reason that many people still believe the redshift
> explains the dark night sky is because Olbers' paradox was rediscovered in
> the 1950s--by steady state cosmologists. In the steady state cosmology,
> the redshift DOES explain the dark night sky. But it is NOT the solution
> in the big bang cosmology.
As to the claims of the steady state theorists. Remember, it was the steady
state theorists who came up with the ideas of nucleosynthesis in stars, which
turned out to free the big bang models of that time from the constraint of
creating heavy elements beyond lithium and berylium. So, just because the ss
theorists came up with redshift/diminishing energy of photons to explain dark
skies doesn't mean the idea should be dismissed.
As I stated, the redshift due to expansion only contributes some of the whole,
the finite age contributes the majority. And, as to absorption, I have not
seen the numbers.
Nice
> Nice wrote:
>
> > The redshift IS irrelevant. If you converted every galaxy redshift
into a
> > blueshift of the same magnitude, the night sky would still be dark. An
> > entire book has been written about Olbers' paradox and discusses wrong
> > solutions (redshift, interstellar absorption, inability to perceive
faint
> > light, etc.) as well as the two correct solutions (finite age of the
> > universe, lack of energy in the universe): DARKNESS AT NIGHT by Edward
> > Harrison.
>
> So, just for arguement sake, this reference is to be believed while that
of
> other astronomers is not.
Harrison's book is devoted solely to Olbers' paradox, is written by the
world's expert on Olbers' paradox--a man who has studied the problem for
decades--and not surprisingly has the correct solutions to Olbers' paradox.
Anyone with an interest in this subject should read the book, which is
readable and nontechnical. Any good library should have it.
> > Historically, the reason that many people still believe the redshift
> > explains the dark night sky is because Olbers' paradox was rediscovered
in
> > the 1950s--by steady state cosmologists. In the steady state
cosmology,
> > the redshift DOES explain the dark night sky. But it is NOT the
solution
> > in the big bang cosmology.
>
> As to the claims of the steady state theorists. Remember, it was the
steady
> state theorists who came up with the ideas of nucleosynthesis in stars,
which
> turned out to free the big bang models of that time from the constraint
of
> creating heavy elements beyond lithium and berylium. So, just because
the ss
> theorists came up with redshift/diminishing energy of photons to explain
dark
> skies doesn't mean the idea should be dismissed.
True. I'm just explaining why, even today, many people think the redshift
explains the dark night sky: because, in the steady state cosmology, it
DOES explain the dark night sky. (An aside: one steady state
theorist--Fred Hoyle--did indeed come up with the idea of nucleosynthesis
in stars, but he says the steady state cosmology did not motivate his
work.)
> As I stated, the redshift due to expansion only contributes some of the
whole,
> the finite age contributes the majority. And, as to absorption, I have
not
> seen the numbers.
The redshift is irrelevant. It only contributes a factor of two or so.
You need a factor of *trillions* to explain the dark night sky. The finite
age of the universe and the lack of energy in the universe explain the
darkness of the night sky. The redshift does not.
Paul Schlyter
>>
>>> Will someone please explain to me in (laymans terms) why with all the
>>> billions and billions of stars in the universe, the sky at night between
>>> the stars is still jet black. Of course I know nothing at all about the
>>> universe, but it would seem to me with all those stars out there
>>> emitting light, that the universe would be very bright instead of dark.
>>> Is it because the universe is so large and distances are so great that
>>> the light becomes so diffused and scattered, that by the time it
>>> reaches us, only faint glimmers of light can be seen? I know this is a
>>> dumb question but it is something that has always bugged me.
>>> Thanks for your responses
>>> Tom Hix
>>
>> light coming from objects at greater and greater distances from us, moving it
>> into wavelength bands we cannot see, and 2) the universe is finite in age, so
>> that there has been insufficient time for light to travel to us for objects at
>> distances greater than about 12 to 13 billion lightyears. The latter is more
>> to the point than the former.
>
> 1. is irrelevant. The redshift also shifts UV, X and gamma wavelengths
> to visible light.
universe - the stars - emit mostly in UV, visible light, and IR. At
12-13 billion lightyears the redshift will be large enough to shift
even the UV into IR, which is invisible to our eyes. At X-ray and Gamma
wavelengths, the universe is a much fainter emitter.
> 2. is the true explanation.
about equally much to the darkness of space.
--
----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pau...@saaf.se paul.s...@ausys.se pa...@inorbit.com
WWW: http://hotel04.ausys.se/pausch http://welcome.to/pausch
Rex Deathstar
light in our eyes' direction count too? Coz I thought that's the reason
the whole sky is bright during daytime, instead of just a superbright
dot called the Sun and all else being dark.
Curiously, many nebulae glow becoz they reflect their star's light right?
Later!
Rex