# Does Space Expand?

by Wallace
Tags: expand, space
 Share this thread:
 P: 989 Cosmology scale factor equation: $$\frac{R_u(t_0)}{R_u(t)} = \frac{a(t_0)}{a(t)} = \frac{T_t}{T_0} = 1 + z$$ Universe total observable radius: $$R_u(t_0) = 4.399 \cdot 10^{26} \; \text{m}$$ Cosmic neutrino background radiation temperature at present: $$T_0 = 1.95 \; \text{K}$$ Cosmic neutrino background radiation temperature at neutrino decoupling time t: $$T_t = 1 \cdot 10^{10} \; \text{K}$$ $$R_u(t) = R_u(t_0) \left( \frac{T_0}{T_t} \right) = 4.399 \cdot 10^{26} \; \text{m} \times \left( \frac{1.95 \; \text{K}}{1 \cdot 10^{10} \; \text{K}} \right) = 8.578 \cdot 10^{16} \; \text{m}$$ Universe total observable radius at neutrino decoupling time t: $$\boxed{R_u(t) = 8.578 \cdot 10^{16} \; \text{m}}$$ Reference: Total amount of energy in the Universe - Orion1 #13 Timeline of the Big Bang - Hadron epoch - Wikipedia Neutrino_decoupling - Wikipedia Cosmic neutrino background - Wikipedia Red shift - Highest redshifts
P: 255
 Quote by Orion1 Universe total observable radius: $$R_u(t_0) = 4.399 \cdot 10^{26} \; \text{m}$$
That figure is for what is observable optically, i.e. the CMBR. We can't observe primordial neutrinos. However, your end result will still be a reasonable rough estimate.
 P: 6 The rate of expansion (hubble constant) of the universe is equivalant to the sun moving 3/4 of a mile farther away in 100 years. Do the math. The solar wind adds solar particles to the solar system and universe. This causes distant objects to appear ever more distant as space becomes more opague. Space is not empty as recent articles have stated.
P: 255
 Quote by dtyarbrough This causes distant objects to appear ever more distant as space becomes more opague.
That makes no sense, you need to explain why space becoming opaque would change the wavelength of a spectral line.
P: 526
 Quote by dtyarbrough The rate of expansion (hubble constant) of the universe is equivalant to the sun moving 3/4 of a mile farther away in 100 years. Do the math. The solar wind adds solar particles to the solar system and universe. This causes distant objects to appear ever more distant as space becomes more opague. Space is not empty as recent articles have stated.
This is so wrong, I have to question if you have a clue about what you're talking about.

First of all, recessional velocity on large scales requires a frame of reference. The distance from this FoR is then multiplied by a scale factor, Hubble's constant. This is shown by Hubble's law:

$$V=H_{0}D$$

Also, expansion only effects very large objects, such as galaxies or super clusters, not the Sun.

Second, Hubble's constant is estimated by WMAP to be 70.8 ± 1.6 (km/s)/Mpc.

So, an object that is very far away will appear to have an extremely high recessional velocity, your calculation was completely made up, and had no basis whatsoever.

Also, the cosmic microwave background provides conclusive evidence the universe is expanding and cooling.
P: 255
 Quote by Mark M This is so wrong, I have to question if you have a clue about what you're talking about.
He doesn't, he is confusing reddening with redshift.

 .. Hubble's law: $$V=H_{0}D$$
Use 1AU for D to get V then multiply by a century and you might get the figure he quoted. Of course that ignores all orbital mechanics and doesn't seem to have any rational connection to the solar wind anyway.
P: 526
 Quote by GeorgeDishman He doesn't, he is confusing reddening with redshift. Use 1AU for D to get V then multiply by a century and you might get the figure he quoted. Of course that ignores all orbital mechanics and doesn't seem to have any rational connection to the solar wind anyway.
I just did the calculation, I doesn't come out to exactly what he said, but it is still very low.

Obviously, someone forgot to tell him that Hubble's law is only used for objects affected by the expansion of space.
P: 255
 Quote by Mark M Obviously, someone forgot to tell him that Hubble's law is only used for objects affected by the expansion of space.
Actually it can be used even for gravitationally bound objects. For example we can measure the motion of the Earth relative to the "fixed stars" so we know its angular velocity and we can estimate the mass of the Sun ignoring Hubble expansion. If we then include expansion, over a short period the Earth would accelerate away from the Sun so to keep the same orbit, we would need to very slightly increase the mass of the Sun. Of course the difference is orders of magnitude less than could be measured but it would be possible to calculate the difference (if I could be bothered).

On a more practical note, I did some work on the Pioneer Anomaly many years ago and although the sign was obviously wrong, I calculated the effect of the Hubble flow when they were proposing a follow-up mission. It turns out to be only about an order of magnitude less than the anomaly so it might be possible to measure it with a carefully designed mission.
 P: 989 Cosmology scale factor equation: $$\frac{R_u(t_0)}{R_u(t)} = \frac{a(t_0)}{a(t)} = \frac{T_t}{T_0} = 1 + z$$ Universe total observable radius: $$R_u(t_0) = 4.399 \cdot 10^{26} \; \text{m}$$ Cosmic gravitational wave background redshift at present: $$z \geq 10^{25}$$ $$R_u(t) \leq \frac{R_u(t_0)}{1 + z} = \frac{4.399 \cdot 10^{26} \; \text{m}}{1 + 10^{25}} = 43.99 \; \text{m}$$ Universe total observable radius at gravitational wave decoupling time t: $$\boxed{R_u(t) \leq 43.99 \; \text{m}}$$ Reference: Cosmic microwave background radiation - Wikipedia Cosmic inflation - Wikipedia Timeline of the Big Bang - Inflationary epoch - Wikipedia Cosmic gravitational wave background - Wikipedia Red shift - Highest redshifts
 P: 989 The the LCDM model scale factor is defined as: $$a(t) = \left[ \frac{\Omega_m}{\Omega_v} \sinh^2 \left( \frac{3}{2} \sqrt{\Omega_v} H_0 t \right) \right]^{\frac{1}{3}}$$ Differentiating the scale factor function with respect to t: $$\frac{d a(t)}{dt} = \frac{d}{dt} \left[ \frac{\Omega_m}{\Omega_v} \sinh^2 \left( \frac{3}{2} \sqrt{\Omega_v} H_0 t \right) \right]^{\frac{1}{3}} = \frac{ \Omega_m H_0 \cosh \left(\frac{3}{2} \sqrt{\Omega_v} H_0 t \right) \sinh \left(\frac{3}{2} \sqrt{\Omega_v} H_0 t \right)}{\sqrt{\Omega_v} \left(\frac{\Omega_m \sinh^2 \left(\frac{3}{2} \sqrt{\Omega_v} H_0 t \right)}{\Omega_v} \right)^{2/3}}$$ The scale factor derivative function: $$\boxed{\frac{d a(t)}{dt} = \frac{ \Omega_m H_0 \cosh \left(\frac{3}{2} \sqrt{\Omega_v} H_0 t \right) \sinh \left(\frac{3}{2} \sqrt{\Omega_v} H_0 t \right)}{\sqrt{\Omega_v} \left(\frac{\Omega_m \sinh^2 \left(\frac{3}{2} \sqrt{\Omega_v} H_0 t \right)}{\Omega_v} \right)^{2/3}}}$$ Is this equation correct? Attachments: plot a(t), plot a'(t) Reference: LambdaCDM geometry - mathematical details - Wikipedia Attached Thumbnails
 P: 255 I'm not good enough at the math to quickly check your solution but I believe there is no analytic solution when there is more than one phase involved. The graph is initially matter dominated but becomes energy dominated so I think you have to perform an integration to get the curve. However, the section "mixtures" here suggests it can be solved: http://en.wikipedia.org/wiki/Friedma...eful_solutions The "Lecture notes on Astrophysics" looks comprehensive too, though beyond my level.
 P: 29 i was thinking about light. a particle and/or a wave ? but what about darkness.? does darkness move at the speed of light? does light move at the speed of darkness? does light really bend around corners or is it pulled around by darkness? in a dark universe, does the universe expand when light appears? does light "push" darkness away ? but what if the universe is in a "bubble"? would the darkness get squashed? is light a constant in the universe.? does the universe expand at different speeds at different times depending on how much is being added at that particular time?
Mentor
P: 11,891
 Quote by lostprophets i was thinking about light. a particle and/or a wave ?
The best description I've heard is that light is an electromagnetic wave that transfers energy only in packets we call photons.

 but what about darkness.? does darkness move at the speed of light? does light move at the speed of darkness?
Darkness is nothing but the absence of light, similar to how a vacuum is the absence of matter in a volume of space.

 does light really bend around corners or is it pulled around by darkness? in a dark universe, does the universe expand when light appears? does light "push" darkness away ?
Light really does diffract (bend if you want to call it that) around corners to a certain extent that depends on the wavelength. The rest of the quote doesn't make any sense.

 but what if the universe is in a "bubble"? would the darkness get squashed? is light a constant in the universe.? does the universe expand at different speeds at different times depending on how much is being added at that particular time?
I think you have a misunderstanding on how we view the universe. I cannot answer these questions because they don't even make sense with current cosmological models. My suggestion is to read up on the subject. There are plenty of websites including wikipedia that will help you understand. Here's two articles that will greatly help you if you read them and follow all the links around. Don't be surprised if it doesn't make much sense first, as unless you understand the basics of light and matter the terms won't mean much.

http://en.wikipedia.org/wiki/Universe
http://en.wikipedia.org/wiki/Physical_cosmology
P: 29
 Quote by Drakkith Darkness is nothing but the absence of light, similar to how a vacuum is the absence of matter in a volume of space. Light really does diffract (bend if you want to call it that) around corners to a certain extent that depends on the wavelength. The rest of the quote doesn't make any sense. http://en.wikipedia.org/wiki/Universe http://en.wikipedia.org/wiki/Physical_cosmology
:o are you serious?
darkness is nothing but the absence of light..... ooosh
so what came first, the darkness or the light?
but i thought we knew or at least thought ,that there is no such thing as nothing
Mentor
P: 11,891
 Quote by lostprophets :o are you serious? darkness is nothing but the absence of light..... ooosh so what came first, the darkness or the light?
Light has been around since the earliest moment of the universe. So I would say light. What happened "before" the universe is pure speculation and doesn't belong here. (Just in case you were going to bring that up)

 but i thought we knew or at least thought ,that there is no such thing as nothing
That is more philosophy than science. We have defined darkness to be the absence of visible light, just as we have defined a vacuum to be the absence of matter.
 P: 989 These are the scale factor equations that I reviewed from reference 1 and 2. Inflation Hubble parameter (end of inflationary epoch): 'ref. 1 p. 34 (167)' $$H_i = \frac{1}{t_{i}} = \frac{1}{10^{-32} \; \text{s}} = 10^{32} \; \text{s}^{-1}$$ $$\boxed{H_i = 10^{32} \; \text{s}^{-1}}$$ Inflation scale factor: 'ref. 1 p.35 (165)' $$a(t) \propto e^{H_i t} \tag{1}$$ Radiation scale factor: 'ref. 1 p. 22 (119)' $$a(t) = (2 H_0)^{\frac{1}{2}} \cdot t^{\frac{1}{2}} \tag{2}$$ Matter scale factor: 'ref. 1 p. 21 (115)' $$a(t) = \left( \frac{4 H_0}{2} \right)^{\frac{2}{3}} \cdot t^{\frac{2}{3}} \tag{3}$$ LCDM matter scale factor: 'ref. 2' $$a(t) = \left[ \frac{\Omega_m}{\Omega_v} \sinh^2 \left( \frac{3}{2} \sqrt{\Omega_v} H_0 t \right) \right]^{\frac{1}{3}} \tag{4}$$ Equations 2 and 3 appear to be describing a universe that is much younger. Attachments: plot 1, plot 2,3,4 Reference: Friedmann equations - useful solutions - Wikipedia Northern Illinois University - Physics 652 - Astrophysics LambdaCDM - geometry - mathematical details - Wikipedia Attached Thumbnails
P: 29
 Quote by Drakkith Light has been around since the earliest moment of the universe. So I would say light. .
i respect your guess.
if light was more abundant than darkness at the start where as the reverse is true now, am i to believe then that the universe is getting smaller?
what if it was expanding and contracting
i ask about light "pushing" darkness (light pressure) clearing a path .
so if we had darkness first with energy, then light energy appears,room has to be made for this light.
could light then clear this "room" creating a vacuum redundant of energy once this light has lost its energy and gone..this then takes time to rebuild itself with dark energy matter, un til it over crowds sparking another light source and repeats the process.

this would mean light energy is finite but that does not mean the universe cannot expand..

i could be way off and have no idea what im on about.but ive read some say that the universe is expanding fast than light... how do we measure this.do we measure it with light?
if light is "pushing" then light will always be behind therefore it could be seen that anything infront of it is moving fast when really its not
P: 255
 Quote by lostprophets i respect your guess. if light was more abundant than darkness at the start where as the reverse is true now, am i to believe then that the universe is getting smaller?
As has been said, darkness is only the absence of light, so in the presence of a single particle of light, the universe is not dark.

 what if it was expanding
It is expanding, and the rate at which it does so is increasing.

 i ask about light "pushing" darkness (light pressure) clearing a path.
That is poetic licence, it has no physical meaning. Light pressure can push a sail around (look up the Ikaros project) but darkness isn't a substance, just the absence of light.

 so if we had darkness first with energy, then light energy appears,
For the first 378,000 years, the whole universe looked like the interior of the Sun, the farther back in time you go, the brighter it was.

 room has to be made for this light.
Space, time and light possibly arose together but we don't know, that is presently beyond our understanding.

 this would mean light energy is finite but that does not mean the universe cannot expand..
It is definitely expanding, it may be finite or infinite, we cannot tell which.

 i could be way off and have no idea what im on about.but ive read some say that the universe is expanding fast than light... how do we measure this.do we measure it with light?
Yes. Surprisingly, the light can still reach us, but I'd need to go into maths to explain why.

 if light is "pushing" then light will always be behind therefore it could be seen that anything infront of it is moving fast when really its not
Light was created everywhere equally and moved in all directions. There was no "in front" or "behind", it always surrounded.

 Related Discussions Special & General Relativity 38 Cosmology 3 General Physics 5 Calculus & Beyond Homework 5 General Physics 5