How exactly does the universe expand?

[Thomas1989]

Good Evening everyone, I have 2 short questions about expansion.

From what I think I have learned based on various posts, expansion really means the distances between remote galaxies that are increasing over time. Am I right? Thank you marcus for the pinned thread about the balloon analogy, that really helped!

If my understanding of expansion is correct, I'd like to know: What exactly is it that causes the expansion? Is it dark energy?

Secondly: As the universe expands and the distance between objects increases, is new space actually created and 'injected' in between them? Or does the space between objects actually 'stretch'? I think I've taken the balloon analogy too literally, to the point where I imagine space behaving like rubber or some other stretchy material. I don't have the faintest idea how it behaves, I'm hoping somebody else might be able to enlighten me.

As always, thank you for your time and patience!

Cheers,
Tom

 

[Mordred]

As I am on my phone atm and I'm feeling lazy atm. I'll post an article myself and other PF members wrote. On expansion and redshift. Its lengthy though lol. So if I rewrite it It will be broken down into three sections.

EXPANSION AND REDSHIFT
1) What is outside the universe?
2) What is causing the expansion of the universe?
3) Is expansion, faster than light in parts of the Universe, and How does this not violate the faster than light speed limit?
4) What do we mean when an object leaves our universe?
5) What do we mean when we say homogeneous and isotropic?
6) Why is the CMB so vital in cosmology?
7) Why is the LambdaCDM so vital to cosmologists?
8) Why are all the galaxies accelerating from us?
9) Is Redshift the same as Doppler shift?
9) How do we measure the distance to galaxies?
10) What is a Cepheid or standard candle

These are some of the common questions I will attempt to address in the following article
First we must define some terms and symbols used.

Planck constant: $h\ =\ 6.62606876(52)\ \times\ 10^{-34}\ J\ s$
Gravitational constant: $G\ =\ 6.673(10)\ \times\ 10^{-11}\ m^{3} kg^{-1} s^{-2}$
Speed of light in a vacuum:$c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}$

The parsec (symbol: pc) is a unit of length used in astronomy, equal to about 30.9 trillion kilometers (19.2 trillion miles). In astronomical terms, it is equal to 3.26 light-years, and in scientific terms it is equal to 3.09×10¹³ kilometers
Mpc=1 million Parsecs

Universe: A generalized definition of the universe can be described as everything that is. In Cosmology the universe can be described as everything measurable in our space-time either directly or indirectly. This definition forms the basis of the observable universe. The Hot Big Bang model does not describe prior to 10^-43 seconds. The LambdaCDM or $\Lambda$CDM model is a fine tuned version of the general FLRW (Freidmann Lemaitre Robertson Walker) metrics, where the six observationally based model parameters are chosen for the best fit to our universe.

The Observable universe is 46 Billion light years, or 4.3×10²⁶ meters with an age as of 2013, is 13.772 ± 0.059 billion years.
In the hot big bang model we do not think of the universe as starting from a singularity (infinitely, hot, dense point) instead measurements agree space-time as simply expanding. That expansion is homogeneous and isotropic. If you were to take a telescope and look at the night sky, no matter where you look the universe looks the same or homogeneous meaning no preferred location. As you change directions with the telescope you will find that no matter which direction you look the universe looks the same or isotropic meaning no preferred direction. These terms in cosmology are only accurate at certain scales. Below 100Mpc it is obvious that the universe is inhomogeneous and anisotropic. As such objects as stars and galaxies reside in this scale. This also tells us that there is no center of the universe, as a center is a preferred location. These terms also describe expansion. Expansion will be covered in more detail in the Cosmological Redshift section. Whether or not the universe is finite or infinite is not known. However if it is infinite now so it must be in the beginning.
Common misconceptions arise when one tries to visualize a finite universe such questions include.

"So how do we see farther than 13.772 billion light years?" The answer lies in expansion; as light is traveling towards us, space-time has expanded.
“If the universe is finite what exists outside the Universe?" If you think about this question with the above definition of the universe you will realize that the question is meaningless. One accurate answer in regards to cosmology is nonexistent.
"What makes up the barrier between our universe and outside our universe?" The short answer is there is no barrier.


The CMB, (Cosmic Microwave Background) The CMB is thermal radiation filling the Observable universe almost uniformly, This provides strong evidence of the homogeneous and isotropic measurements and distances. As the universe expanded, both the plasma and the radiation filling it grew cooler. When the universe cooled enough, protons and electrons combined to form neutral atoms. These atoms could no longer absorb the thermal radiation, and so the universe became transparent instead of being an opaque fog. Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. CMB photons were emitted at about 3000 Kelvin and are now 2.73 Kelvin blackbody radiation. Their currently observed energy is 1/1000th of their energy as emitted.

In order to measure an objects motion and distance in cosmology it is important to properly understand redshift, Doppler shift and gravitational redshift. Incorrect usage of any of these can lead to errors in our measurements.

Doppler shift and redshift are the same phenomenon in general relativity. However you will often see Doppler factored into components with different names used, as will be explained below. In all cases of Doppler, the light emitted by one body and received by the other will be red or blueshifted i.e. its wavelength will be stretched. So the color of the light is more towards the red or blue end of the spectrum. As shown by the formula below.

$$\frac{\Delta_f}{f} = \frac{\lambda}{\lambda_o} = \frac{v}{c}=\frac{E_o}{E}=\frac{hc}{\lambda_o} \frac{\lambda}{hc}$$

The Cosmological Redshift is a redshift attributed to the expansion of space. The expansion causes a Recession Velocity for galaxies (on average) that is proportional to DISTANCE.
A key note is expansion is the same throughout the cosmos. However gravity in galaxy clusters is strong enough to prevent expansion. In other words galaxy clusters are gravitationally bound. In regards to expansion it is important to realize that galaxies are not moving from us due to inertia, rather the space between two coordinates are expanding. One way to visualize this is to use a grid where each vertical and horizontal joint is a coordinate. The space between the coordinates increase rather than the coordinates changing. This is important in that no FORCE is acting upon the galaxies to cause expansion. As expansion is homogeneous and isotropic then there is no difference in expansion at one location or another. In the $\Lambda$CDM model expansion is attributed to the cosmological constant described later on. The rate a galaxy is moving from us is referred to as recession velocity. This recession velocity then produces a Doppler (red) shift proportional to distance (please note that this recession velocity must be converted to a relative velocity along the light path before it can be used in the Doppler formula). The further away an object is the greater the amount of redshift. This is given in accordance with Hubble’s Law. In order to quantify the velocity of this galactic movement, Hubble proposed Hubble's Law of Cosmic Expansion, aka Hubble's law, an equation that states:

Hubble’s Law: The greater the distance of measurement the greater the recessive velocity

Velocity = H₀ × distance.

Velocity represents the galaxy's recessive velocity; H₀ is the Hubble constant, or parameter that indicates the rate at which the universe is expanding; and distance is the galaxy's distance from the one with which it's being compared.

The Hubble Constant The Hubble “constant” is a constant only in space, not in time,the subscript ‘0’ indicates the value of the Hubble constant today and the Hubble parameter is thought to be decreasing with time. The current accepted value is 70 kilometers/second per mega parsec, or Mpc. The latter being a unit of distance in intergalactic space described above.
Any measurement of redshift above the Hubble distance defined as H₀ = 4300±400 Mpc will have a recessive velocity of greater than the speed of light. This does not violate GR because a recession velocity is not a relative velocity or an inertial velocity. It is precisely analogous to a separation speed. If, in one frame of reference, one object is moving east at .9c, and another west at .9c, they are separating by 1.8c. This is their recession velocity. Their relative velocity remains less than c. In cosmology, two things change from this simple picture: expansion can cause separation speeds much greater even than 2c; and relative velocity is not unique, but no matter what path it is compared along, it is always less than c, as expected.

z = (Observed wavelength - Rest wavelength)/(Rest wavelength) or more accurately

1+z= λ_observed/λ_emitted or z=(λ_observed-λ_emitted)/λ_emitted

$$1+Z=\frac{\lambda}{\lambda_o}$$ or $$1+Z=\frac{\lambda-\lambda_o}{\lambda_o}$$

λ₀= rest wavelength
Note that positive values of z correspond to increased wavelengths (redshifts).
Strictly speaking, when z < 0, this quantity is called a blueshift, rather than
a redshift. However, the vast majority of galaxies have z > 0. One notable blueshift example is the Andromeda Galaxy, which is gravitationally bound and approaching the Milky Way.
WMAP nine-year results give the redshift of photon decoupling as z=1091.64 ± 0.47 So if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years away.

Cosmological Constant is a homogeneous energy density that causes the expansion of the universe to accelerate. Originally proposed early in the development of general relativity in order to allow a static universe solution it was subsequently abandoned when the universe was found to be expanding. Now the cosmological constant is invoked to explain the observed acceleration of the expansion of the universe. The cosmological constant is the simplest realization of dark energy, which the more generic name is given to the unknown cause of the acceleration of the universe. Indeed what we term as "Dark" energy is an unknown energy that comprises most of the energy density of our cosmos around 73%. However the amount of dark energy per m³ is quite small. Some estimates are around about 6 × 10^-10 joules per cubic meter. However their is a lot of space between large scale clusters, so that small amount per m³ adds up to a significant amount of energy in total. In the De_Sitter FLRW metric (matter removed model)
this is described in the form.

H_o$\propto\sqrt\Lambda$

Another term often used for the cosmological constant is vacuum energy described originally by the false vacuum inflationary Model by A.Guth. The cosmological constant uses the symbol Λ, the Greek letter Lambda.
The dark energy density parameter is given in the form:
$\Omega_\Lambda$ which is approximately 0.685

The Doppler Redshift results from the relative motion of the light emitting object and the observer. If the source of light is moving away from you then the wavelength of the light is stretched out, i.e., the light is shifted towards the red. When the wavelength is compressed from an object moving towards you then it moves towards the blue end of the spectrum. These effects, individually called the blueshift and the redshift are together known as Doppler shifts. The shift in the wavelength is given by a simple formula

(Observed wavelength - Rest wavelength)/(Rest wavelength) = (v/c)

$$f=\frac{c+v_r}{c+v_s}f_o$$

c=velocity of waves in a medium
$$v_r$$ is the velocity measured by the source using the source’s own proper-time clock(positive if moving toward the source
$$v_s$$ is the velocity measured by the receiver using the source’s own proper-time clock(positive if moving away from the receiver)

The above are for velocities where the source is directly away or towards the observer and for low velocities less than relativistic velocities. A relativistic Doppler formula is required when velocity is comparable to the speed of light. There are different variations of the above formula for transverse Doppler shift or other angles. Doppler shift is used to describe redshift due to inertial velocity one example is a car moving away from you the light will be redshifted, as it approaches you the light and sound will be blueshifted. In general relativity and cosmology, there is a fundamental complication in this simple picture - relative velocity cannot be defined uniquely over large distances. However, it does become unique when compared along the path of light. With relative velocity compared along the path of the light, the special relativity Doppler formula describes redshift for all situations in general relativity and cosmology. It is important to realize that gravity and expansion of the universe affect light paths, and how emitter velocity information is carried along a light path; thus gravity and expansion contribute to Doppler redshift

Gravitational Redshift describes Doppler between static emitter and receiver in a gravitational field. Static observers in a gravitational field are accelerating, not inertial, in general relativity. As a result (even though they are static) they have a relative velocity in the sense described under Doppler. Because they are static, so is this relative velocity along a light path. In fact, the relative velocity for Doppler turns out to depend only on the difference in gravitational potential between their positions. Typically, we dispense with discussion of the relative velocity along a light path for static observers, and directly describe the resulting redshift as a function of potential difference. When the potential increases from emitter to receiver, you have redshift; when it decreases you have blue shift. The formula below is the gravitational redshift formula or Einstein shift off the vacuum surrounding an uncharged, non rotating, spherical mass.
$$\frac{\lambda}{\lambda_o}=\frac{1}{\sqrt{(1 - \frac{2GM}{r c^2})}}$$

G=gravitational constant
c=speed of light
M=mass of gravitational body
r= the radial coordinate (measured as the circumference, divided by 2pi, of a sphere centered around the massive body)

The rate of expansion is expressed in the $\Lambda$CDM model in terms of
The scale factor, cosmic scale factor or sometimes the Robertson-Walker scale factor parameter of the Friedmann equations represents the relative expansion of the universe. It relates the proper distance which can change over time, or the comoving distance which is the distance at a given reference in time.

d(t)=a(t)d_o

where d(t) is the proper distance at epoch (t)
d₀ is the distance at the reference time (t_o)
a(t) is the comoving angular scale factor. Which is the distance coordinate for calculating proper distance between objects at the same epoch (time)
r(t) is the comoving radial scale factor. Which is distance coordinates for calculating proper distances between objects at two different epochs (time)

$$Proper distance =\frac{\stackrel{.}{a}(t)}{a}$$

The dot above a indicates change in.

the notation R(t) indicates that the scale factor is a function of time and its value changes with time. R(t)<1 is the past, R(t)=1 is the present and R(t)>1 is the future.

$$H(t)=\frac{\stackrel{.}{a}(t)}{a(t)}$$

Expansion velocity
$$v=\frac{\stackrel{.}{a}(t)}{a}$$

This shows that Hubble's constant is time dependant.



Cosmic Distance ladder, also known as Extragalactic distance scale. Is easily thought of as a series of different measurement methods for specific distance scales. Previous in the article we discussed the various forms of Redshift. These principles are used in conjunction with the following methods described below. Modern equipment now allows use spectrometry. Spectrographs of an element give off a definite spectrum of light or wavelengths. By examining changes in this spectrum and other electromagnetic frequencies with the various forms of shifts caused by relative motion, gravitational effects and expansion. We can now judge an objects luminosity where absolute luminosity is the amount of energy emitted per second.

Luminosity is often measured in flux where flux is

$$f=\frac{L}{4\pi r^2}$$

However cosmologists typically use a scale called magnitudes. The magnitude scale has been developed so that a 5 magnitude change corresponds to a differents of 100 flux.
Rather than cover a large range of those distance scales or rungs on the ladder I will cover a few of the essential steps to cosmological distance scales. The first rung on the ladder is naturally.

Direct measurements: Direct measurements form the fundamental distance scale. Units such as the distance from Earth to the sun that are used to develop a fundamental unit called astronomical unit or AU. During the orbit around the sun we can take a variety of measurements such as Doppler shifts to use as a calibration for the AU unit. This Unit is also derived by a method called Parallax.

Parallax. Parallax is essentially trigonometric measurements of a nearby object in space. When our orbit forms a right angle triangle to us and the object to be measured
With the standardized AU unit we can take two AU to form the short leg. With the Sun at a right angle to us the distance to the object to be measured is the long leg of the triangle.

Moving Cluster Parallax is a technique where the motions of individual stars in a nearby star cluster can be used to find the distance to the cluster.

Stellar parallax is the effect of parallax on distant stars . It is parallax on an interstellar scale, and allows us to set a standard for the parsec.

Standard candles A common misconception of standard candles is that only type 1A supernova are used. Indeed any known fundamental distance
measurement or stellar object
whose luminosity or brightness is
known can be used as a standard
candle. By comparing an objects
luminosity to the observed brightness we can calculate the distance to an object using the inverse square law. Standard candles include any object of known luminosity, such as Cepheid’s, novae, Type 1A supernova and galaxy clusters.

My thanks to the following Contributors, for their feedback and support.

PAllen
Naty1
Jonathon Scott
marcus

Article by Mordred, PAllen

[/QUOTE]

 

[Thomas1989]

Thank you so much Mordred! That is extremely helpful, even if it's way beyond my level of understanding, I'm a patient guy... I'll decipher what I can from it. Cheers!

 

[marcus]

Good Evening everyone, I have 2 short questions about expansion.

From what I think I have learned based on various posts, expansion really means the distances between remote galaxies that are increasing over time. Am I right? Thank you marcus for the pinned thread about the balloon analogy, that really helped!

If my understanding of expansion is correct, I'd like to know: What exactly is it that causes the expansion? Is it dark energy?

...

Thanks for the appreciative words about that pinned thread, quite a few people have contributed to it by now.
Cosmologists have been unlucky in the choice of some of their terminology. "Big Bang" seems to give public a misleading mental image, it is just the start of expansion and they're still investigating and trying to fit testable models to that part of universe history. No consensus as yet.

"Dark energy" is another unfortunate phrase. It's being used as an alias for a curvature constant traditionally denoted by a greek letter Lambda we've had around for around 95 years, as a naturally occurring term in the Einstein GR equation. There is no observational evidence that it corresponds to an actual ENERGY. On the contrary, the last 5 years or so of evidence points to it being a simple constant. So in the professional literature authors often do not say "dark energy"---nowadays they often simply say "cosmological constant" without reference to energy.

This constant had a negligible effect on early history of universe. It began having a noticeable effect only after a few billion years. Until around year 7 billion the expansion of distances was slowing down, and now it is only just barely speeding up.

Lambda is a very slight curvature and it was dwarfed, totally overwhelmed, by all the stuff going on earlier. So the answer to your question "Was it dark energy?" is NO, not if by "dark energy" you mean what people usually mean by it namely the cosmological constant.

There may have been some actual energy field, long since decayed, that was at work in the early universe and which participated in kicking off the expansion! We can speculate! Maybe a big brother of the Higgs field! The question itself "What started expansion, right at the start?" is an exciting question. Various ideas have been proposed. Some people are working on models where there is a bounce. Quantum effects make gravity repellent at extremely high density, so a prior contraction rebounds, starting the expansion. There is no professional consensus so far.

One thing they do agree on is that classical 1915 Einstein GR equation explains how expansion once started will continue without constant input of energy. The equation only allows for a gradual predictable change in the rate. There is one enormously difficult to accept, but apparently true, thing about cosmology and that is geometry has a mind of its own. It is not like what Euclid said, it is not fixed static with triangles always adding to 180 degrees. Geometry RESPONDS to the flow of matter and to its own past. If it gets started growing, it will continue at least for a while. If it gets started bending (say because of a flow of some matter) it will continue at least for a while. And bending makes triangles add up to something besides 180.

We have to accept this because it is also our law of gravity, that turned out to be more successful than Newtons. Gravity=dynamic geometry. We don't have anything better than this at present. It has been tested a lot, in all sorts of ways, at many different scales (earth, solar system, other stars, distorted lensing effects of clusters of galaxies and unseen clouds, and (yes) expansion of distance. It is all part of the same thing, the same simple equation that WORKS. And it is the best law of gravity we have so far. So it puts our intuitions in a bind. Our intuitions say that geometry cannot be dynamic and influenced by flows of matter, it has to be fixed exactly the way Euclid said.

That is the predicament we are in, our minds are in. Space is not a substance IN geometry. It IS geometry. It does not require "energy" to make geometry change. Geometry has an equation (a "mind of its own") describing how it behaves==an equation which is also the simplest most correct law of gravity we have so far. We don't have an explanation for why this equation (although Ted Jacobson, in a famous 1995 paper came close to explaining--made real progress) and so so we just have to accept it for the time being. With our 2000 year old Euclidean intuitions screaming in pain.

I think this is is one reason people fall into the verbal trap of calling what is really a naturally occurring (automatically occurring) constant in the Einstein equation by the name "dark energy".
Our intuition is taking revenge on us for all the indignities it has suffered over the past 100 years.

for an article debunking the "mysterious dark energy" buzz-think
http://arxiv.org/abs/1002.3966
or if you don't have the link handy, google "rovelli prejudices". It will be the top hit with that two-word search.

 

[phinds]

I think I've taken the balloon analogy too literally ...

the balloon analogy is a good tool but it has its problems, as explained here:

www.phinds.com/balloonanalogy

 

[Thomas1989]

Thanks for the appreciative words about that pinned thread, quite a few people have contributed to it by now.

Well deserved! I might have mentioned it in another thread, but I always look forward to reading your posts especially, as well as several others here - so thanks for all your help.

I'm reading about the cosmological constant now as I'm extremely new to all this. The terminology confuses me more than the math (okay, that's a lie!) but as you say, unlucky! I recently read about the experiments they are conducting deep underground in mines and so on, trying to detect dark matter or WIMPS. The idea of it being a particle, or discovering something exotic like a table of dark elements was firmly entrenched in my mind.

 

[marcus]

Well deserved! I might have mentioned it in another thread, but I always look forward to reading your posts especially, as well as several others here - so thanks for all your help.

I'm reading about the cosmological constant now as I'm extremely new to all this. The terminology confuses me more than the math (okay, that's a lie!) but as you say, unlucky! I recently read about the experiments they are conducting deep underground in mines and so on, trying to detect dark matter or WIMPS. The idea of it being a particle, or discovering something exotic like a table of dark elements was firmly entrenched in my mind.

There's no evidence that "dark energy" (the slight intrinsic spacetime curvature) is related to DARK MATTER.

There are half a dozen good reasons supporting the dark matter hypothesis. It condenses into largescale structure at a different rate from ordinary matter, and would tend to dominate structure formation. Computer simulations with DM qualitatively match observed structure.

We can SEE clouds of DM, where it is denser, by how it optically distorts the galaxies behind the clouds. So density maps (like contour maps) have been made. Very interesting (called weak gravitational lensing.) Clusters of galaxies (of ordinary matter) tend to be surrounded by these clouds.

The mechanism by which DM condenses into largescale structure does not allow it to curdle into small scale structure the way OM does. Can't get hot and blow off surplus kinetic energy like OM can.
Pretty clearly it is a good hypothesis, so it makes sense for people to make detectors (in space, or down in mines, etc.) to try to detect DM particles. Best wishes to them! Confirmed detection sometime in next 10 years? By 2023 maybe? Would be a major triumph, perhaps the most important physics/astrophysics discovery of the decade.

But that is not "dark ENERGY". DM gathers in huge wisps and clouds by its own gravity. The small intrinsic curvature of space-time is a CONSTANT, the same everywhere at all times. Very different.

 

[Mordred]

Well deserved! I might have mentioned it in another thread, but I always look forward to reading your posts especially, as well as several others here - so thanks for all your help.

I'm reading about the cosmological constant now as I'm extremely new to all this. The terminology confuses me more than the math (okay, that's a lie!) but as you say, unlucky! I recently read about the experiments they are conducting deep underground in mines and so on, trying to detect dark matter or WIMPS. The idea of it being a particle, or discovering something exotic like a table of dark elements was firmly entrenched in my mind.

That was one of the reasons of the sheer size of the article. I felt it was important to describe each term or constant as they showed up in the article. However to save space some of the
descriptives were kept brief

Also thank you for the appreciation of the article its nice to get positive feedback from the intended audience

 

[marcus]

Well deserved! I might have mentioned it in another thread, but I always look forward to reading your posts especially, as well as several others here - so thanks for all your help.

I'm reading about the cosmological constant now as I'm extremely new to all this. The terminology confuses me more than the math (okay, that's a lie!) but as you say, unlucky! I recently read about the experiments they are conducting deep underground in mines and so on, trying to detect dark matter or WIMPS. The idea of it being a particle, or discovering something exotic like a table of dark elements was firmly entrenched in my mind.

Thanks! Your comments are heartening. Here's an interesting passage from page 2 of that paper by Bianchi and Rovelli you can get by googling "rovelli prejudices"

==quote==
In fact, it may not even be true that Einstein introduced the λ term because of cosmology. He probably knew about this term in the gravitational equations much earlier than his cosmological work. This can be deduced from a footnote of his 1916 main work on general relativity [9] (the footnote is on page 180 of the English version). Einstein derives the gravitational field equations from a list of physical requirements. In the footnote, he notices that the field equations he writes are not the most general possible ones, because there are other possible terms. The cosmological term is one of these (the notation “λ” already appears in this footnote).
The most general low-energy second order action for the gravitational field, invariant under the relevant symmetry (diffeomorphisms) is

S[g] = (1/16πG)∫ (R[g] − 2λ)√g

which leads to (1). It depends on two constants, the Newton constant G and the cosmological constant λ, and there is no physical reason for discarding the second term.
From the point of view of classical general relativity, the presence of the cosmological term is natural and a vanishing value for λ would be more puzzling than a finite value: the theory naturally depends on two constants; the fact that some old textbooks only stress one (G) is only due to the fact that the effects of the second (λ) had not been observed yet.
==endquote==

The basic reason is that in physics standard practice is to include all terms allowed by the symmetry of the theory. The underlying symmetry of GR is the diffeomorphism group, that is, "general covariance" as Einstein called it. This more than anything else dictates the nature of space-time. The differ group (general covariance) only allows TWO gravitational constants, G and Lambda. So the natural thing is to include both in the equation and expect both to play a role.
A priori there was no reason to assume that one of the two naturally occurring constants would vanish, be exactly equal to zero. However no effect of Lambda was observed until 1998.
So this constant of nature was a "sleeper" for over 80 years
http://arxiv.org/pdf/1002.3966v3.pdf
Check the article out! It is written for wide audience and there are plenty more goodies like the above passage.

 

[Mordred]

That dark energy debunk article was an enjoyable reading. Thanks for posting it

 

[Chalnoth]

If my understanding of expansion is correct, I'd like to know: What exactly is it that causes the expansion? Is it dark energy?

Well, the interaction between gravity and the contents of the universe determine how the expansion changes with time. What caused the expansion to start in the first place is whatever physics set up our universe in the first place. And we don't yet know the answer to that.

Secondly: As the universe expands and the distance between objects increases, is new space actually created and 'injected' in between them?

Yes, new space is created by the expansion. Stretching would imply that there is some sense in which space resists the expansion, or that it becomes somehow thinner after expanding. Neither is the case.

 

[Mordred]

Yes, new space is created by the expansion. Stretching would imply that there is some sense in which space resists the expansion, or that it becomes somehow thinner after expanding. Neither is the case.

That is a great correlation, never thought of it before but makes
sense. Thanks for that

Not to imply I thought of space stretching. Your straight forward explanation cuts to the chase so to speak.

 

[marcus]

AAARGH! more verbal trouble.

a stretch limo is not any thinner, it is just lengthened
a Boeing 747 stretch airliner is not any narrower, it is just elongated

whether or not the thinning down idea is implied by the word, or just elongation, depends on the context, I think.

I like the word stretch for the elongation of wavelength and of distance because it is one syllable and familiar-sounding. So if it is all right with you all I will continue using it with the understanding that no thinning is implied when I apply it to distances and wavelengths.

 

[timmdeeg]

Yes, new space is created by the expansion. Stretching would imply that there is some sense in which space resists the expansion, or that it becomes somehow thinner after expanding. Neither is the case.

I would like to ask whether one should think of this statement "new space is created" as a helpful notion or perhaps as an interpretation of the cosmological redshift, rather than of a physical fact, which is measurable.

Because remembering the article
Expanding Space: the Root of all Evil? which quotes Weinberg

an interested student and reader of New Scientist may have seen Martin Rees & Steven Weinberg (1993) state

... how is it possible for space, which is utterly empty, to expand? How can nothing expand? The answer is: space does not expand. Cosmologists sometimes talk about expanding space, but they should know better.

while being told by Harrison (2000) that

expansion redshifts are produced by the expansion of space between bodies that are stationary in space.

I feel tempted to add, how can nothing increase?

Nevertheless, I have some hope, that it is somehow compelling to talk about the creation of space, though knowing that it isn't a substance. How should a layman understand the true physical fact of increasing distances otherwise in terms of geometry merely? Without taking reference to increasing space?

 

[ImaLooser]

I feel tempted to add, how can nothing increase?

I don't know, but I don't see why it shouldn't. I don't see why there couldn't be an infinite "supply" of nothing. Nothing could be produced at no cost. I don't see what the problem is, if that is what the universe feels like doing.

 

[timmdeeg]

I don't know, but I don't see why it shouldn't.

Well, I can measure that something increases, but nothing? Or how would you describe the difference of state before and after nothing has increased?

 

[Naty1]

Hi Thomas,

As the universe expands and the distance between objects increases, is new space actually created ...

is an interesting question without firm answers. That's because nobody really knows "what is space? What's the difference between 'space' and 'distance'. Is space continuous [as in GR] or discrete [as in QM]? Where does it come from? Where does it go?" We've discussed these kind of questions...and yours...so if you search you'll find many good ideas in these forums.

[For me it's hard to figure space is 'nothing' because changing geometry over time [like cosmological expansion] results in particle creation...[also discussed in these forums.]

In the article linked to above [Prejudices...] they write:

...We have an observed physical phenomenon (the accelerated expansion). A simple physical theory explains it (general relativity with nonvanishing λ . However, particle physics claims that it can provide an independent understanding of the phenomenon (a cosmologi-
cal term entirely produced by vacuum fluctuation)...

So we think the universe is expanding and that the expansion is currently accelerating based on generally agreed upon observations.
I don't think GR does 'explain' it...but it at least provides a model that predicts it. 'dark energy'
vacuum energy provides a potentially finer grain of understanding, but that may not be the correct understanding.

Also, keep in mind neither GR nor QM work perfectly...they don't handle BH and Big Bang'singularities'...so we have more to learn.

and while we are at it, use caution regarding 'cosmological distances'...that's also not so easy
to understand as you might think. We all need to develop approaches to interpreting what THAT means.

If you are familiar with Newtonian kinetic and potential energy, no GR, no tensors,etc, you might find Leonard Susskind's third Cosmology tutorial very interesting...on Youtube. It provides lots of basic insights to your questions by showing how some simplified math underlies our understanding of expansion. Susskind even does a simple calculation from scale factors [expansion rates] to show that around year 7 billion the expansion of distances which was slowing down began to speed up...