Dark energy

[Primarygun]

What is dark energy?

Why it is so mysterious for human?

[ydoaPs]

try astronomy/cosmology forum

there are a few threads in there. read them

[Cap'n Refsmmat]

Why is it so mysterious? We don't know what it is!

[noz92]

no one knows for sure, but at the top of this page is see an advertisement thing that says that dark energy and dark matter explained by quantum physics theory.

 

i have an idea of my own that says dark energy and matter are really just what matter and energy are actually the decay of regular matter and enery as we know them as. and that since matter is constantly growing older, and matter is constantly becoming dark matter, the universe is constantly expanding.

 

dark energy and dark matter are what we think, to be what is causing the universe to expand, and what will eventually cause the universal implosion known as the big crunch, so its probably pretty massive.

[Primarygun]

thanks

[Martin]

It may help visualize dark energy to know how much of it is in a cubic km.

It is about half a joule---more exactly 0.6 joule

 

a joule is the amount of energy it takes to lift a kilogram up by about 10 centimeters.

 

it is less than the amount of energy involved in lifting a pound weight up one foot---but in the same ballpark.

 

so a joule is very roughly a harmless smallish amount of energy like a footpound.

 

And the dark energy, by the best measurements so far, in a cubic kilometer is about 0.6 joule.

 

It is supposed to be constant and uniformly distributed. there's other stuff but that's a start

[ydoaPs]

dark energy may not exist. http://www.scienceforums.net/forums/showthread.php?t=4747&page=1

[drz]

dark energy and dark matter are what we think, to be what is causing the universe to expand, and what will eventually cause the universal implosion known as the big crunch

 

I thought in the increasing acceleration scenario the universe would go through a "Big Rip" where even matter is basically shredded at the molecular level.

[Primarygun]

dark matter is the thing which does not follow the law of physics.

So what's dark energy>?

Is it the energy we can't see?

[ydoaPs]

how doesn't dark matter follow the laws of physics?(and which ones?)

 

dark energy is the modern version of the cosmological constant.

[TheProphet]

how doesn't dark matter follow the laws of physics?(and which ones?)

 

dark energy is the modern version of the cosmological constant.

 

Nope.. Dark energy whas prupopsed in order too explain the previuos unknown criterias of the universe! That it was much older and epxanding faster that we thought. This occured during distant Supernovae observations.

The cosmological constant was Einsteins way of maintaining a finite and "resting"(bad word) universe. So too draw that line = is to not have done your homework.

[ydoaPs]

the cosmological constant was einstien's antigravitaional force to keep the universe static. dark energy is the MODERN version of it in which it is much stronger making up for the extra speed. in the future, read a post before you comment on it.

[Sayonara]

the cosmological constant was einstien's antigravitaional force to keep the universe static. dark energy is the MODERN version of it in which it is much stronger making up for the extra speed. in the future, read a post before you comment on it.

http://www.scienceforums.net/forums/showthread.php?t=5368

[TheProphet]

the cosmological constant was einstien's antigravitaional force to keep the universe static. dark energy is the MODERN version of it in which it is much stronger making up for the extra speed. in the future, read a post before you comment on it.

 

Hope u read Martins thread... But look at your post again... U still wrote it as the two ARE the SAME. And that it's an absolut fact, and here u still keep poundering it! Quite some difference between the two as Martin showed.

[Alexa]

Dark matter makes up about 23 percent of the universe's mass-energy budget. Normal matter, the stuff of stars, planets and people, contributes just 4 percent. The rest of the universe is driven by an even more mysterious thing called dark energy.

 

"I'm as big a fan of dark matter and dark energy as anybody else," says astronomer Richard Ellis of Caltech. But, he adds, "I find it very worrying that you have a universe where there are three constituents, of which only one [i.e., ordinary matter] is really physically understood."

 

"When you teach undergraduates, and they say, 'Well, what is dark matter?' Well, nobody's really sure. 'What is dark energy?' We're even less sure. So you have to explain to a student, that … 90 percent of the universe, 95 percent, is in two ingredients that nobody really understands," says Ellis. "This isn't really progress."

 

No one argues that dark energy is difficult to comprehend. And as Trimble points out, it is hardly the first strange idea scientists have had to accept.

 

"It took two generations for people to be comfortable with quantum mechanics," she says. "The fact that you do not have good intuition about [dark energy] is true for quantum mechanics, general relativity, and lots of other things, because we can’t easily mock them up in the laboratory."

 

And for cosmologists, dark energy has solved at least one cosmological conundrum raised by studies of the Cosmic Microwave Background, or CMB.

 

from the article : Dark enery : Astronomers Still 'Clueless' About Mystery Force Pushing Galaxies Apart, by Andrew Chaikin (http://www.space.com)

[ydoaPs]

i am addicted to Alexa's avatar

[drz]

not worried about your avatar being jealous?

[Alexa]

Yeah, I'm adicted to my avatar, too.

 

What about dark enery ?

[Martin]

 

What about dark energy ?

 

there's the thought on some people's minds that it is just another universal constant' date=' like the speed of light

 

 

over the past couple of centuries people have come to accept

a certain number of constants that just are

 

no one has offered a good explanation for why they are what they are or for why we should expect them to be constant everywhere and always

(it's been more of process of getting used to them and beginning to trust them, and there still are doubts that surface now and then about whether such and such really is constant)

 

Lambda might be one of the short list of universal fundamental constants like

c, Planck's hbar, Newton's G

these three are built-in quantitites or proportions in nature that most people most of the time simply ignore or take for granted. they dont ask

 

What about the speed of light ?

 

What about G, the strength of gravity's coupling with energy?

 

What about hbar, linking (among others) vibration and energy ?

 

Lambda could be a residual curvature that spacetime simply has and that is not caused by anything (doesnt correspond to any energy including "dark energy" but is just intrinsic). Could be.

[Martin]

Planck first proposed there was this built-in proportion in nature called hbar in 1899

It took 25 years or so for people to get used to the idea

(recognizable quantum mechanics of Heisenberg and Schroedinger goes back to about 1925)

 

people can quibble about the exact number of years but the point is it took a while to soak in.

 

Lambda appeared in some paper of Einstein around 1917, I think. But that doesnt mean people accepted it. I think until 1998 almost everybody either ignored it or believed it was zero.

 

then in 1998 a positive Lambda was actually observed (the supernovas, everybody here knows the story)

 

I'm inclined to think that it will take a couple of decades to soak in ( that we have a new constant---if in fact we do: it is only a maybe), and to get assimilated

[Martin]

It is a bad idea to call Lambda a "force"

in physics, force is not the same as energy

and curvature is not the same as force

 

in some situations a concentration of energy (say in the form of a dense mass)

can cause a curvature

and that curvature might be felt as a force

but it gets confusing if people simply equate the terms

 

A curvature is the reciprocal of an area

here are some curvatures:

"one over a square mile"

"one over a square yard"

 

this is one of the outrages perpetrated by mathematicians

 

A speed is an understandable thing: a distance over a time:

"one mile over one minute"

"one yard per second"

 

The way curvature is measured is much less understandable

to non-mathematicians.

 

a very small curvature is one over a very large area

 

here is a small curvature:

"one over a square lightyear"

 

here is a very small curvature:

"one over a million square lightyears"

 

the cosmological constant Lambda is an even smaller curvature

than that. I'm skeptical when people talk as if it is easy to

understand. It's hard to picture that small a curvature and

to imagine that it has any significance. It just isn't easy to understand.

 

try to picture a square which is 9.5 billion lightyears on a side.

that is some area

the cosmological constant is one over that area.

[Alexa]

there's the thought on some people's minds that it is just another universal constant, like the speed of light

 

Martin,

 

Even the smartest astronomers were't able to understand the dark energy. So, we may be forgiven about that. I know, Einstein had tried to explain it. Maybe, he just didn't have the time to finish his research.

 

Astronomers expect with WMAP to understand better the expansion of the Universe and the role of dark enery.

 

David Spergel, a Princeton Univ. and a member of the WMAP science team said about it :

 

Dark energy, whatever it is, is something that is not attracted by gravity on the large scales probed by the Sloan Digital Sky Survey.

 

I've got a nice picture about the composition of the universe made up of dark energy, dark matter, and ordinary matter, but I have no idea how to insert it in this post. As you can see, I'm new on this forum, so I have to learn how to navigate through first.

 

Alexa

[Martin]

Martin' date='

 

Even the smartest astronomers were't able to understand the dark energy. So, we may be forgiven about that. [/quote']

 

thats for sure

you seem pretty sophisticated about it so I will tell you my own take:

now is the wrong time to fall in love with one particular picture IMO

 

at first, when I first heard about it (sometime after 1998) I took it to be a kind of evenly distributed energy----like the postulated vacuum energy.

 

in that case it has to be about 0.6 joules per cubic kilometer

 

(also like your guy says, it cannot clump: it cannot attract itself and cause itself to gather in some patches of higher concentration---it has to stay spread out evenly)

 

for several years I believed this picture, which works fairly well to explain the observed curvature

 

then, this year, I saw that a group of researchers at Univ. Waterloo in Ontario had begun a new approach

 

It happens that Ned Wright, a prominent member of the WMAP science team has not yet caught onto this approach AFAIK. Quite possibly David Spergel of the WMAP team also has not (I do not know his work separately).

I respect Ned Wright a lot and have used his website and lecture notes and FAQ repeatedly. But I'm not going to simply follow his lead---which would be to remain skeptical and open and reserve judgement and not flirt too much with new approaches.

 

I am really interested in this new approach by the people at Waterloo and Perimeter Institute. It has to do with bringing together two things: the cosmological constant lambda (the curvature quantity) and the galaxy rotation curve data associated with the "MOND" concept. It has to do with rethinking it so you can get rid of both dark energy and dark matter---kill two mystery birds with one stone---using a single idea these people are in process of working out.

 

I dont expect the rest of the cosmology community to take to this

any time soon, and the initiative may not pan out anyway. But I am watching the developments at Waterloo-Perimeter with considerable interest.

 

the point is that the hypothetical "dark matter" is based on the appearance of a certain acceleration-limit in the circulation of stars in galaxies, below which the acceleration doesnt taper off as fast as expected.

this has caused people to imagine there is invisible extra mass clumped in galaxies. the conventional mainstream cosmologist does not use the

cosmological constant to explain this. he or she postulates "dark matter"

to expain it

 

then there is the "dark energy" which is postulated to explain another observation---largescale acceleration of expansion, not on the scale of a single galaxy but on the universe-wide scale

 

AFAIK the only people on the planet currently working on a concept where one quantity governs both these observed things are the bunch at Perimeter and close associates at a few other campuses----odd places like Warsaw Poland and Montpellier France.

 

I will tell you what the main clue is, that DE and DM are related and you may be able to get rid of both. But first I want to check Ned Wright's website and see if he says anything about MOND. He teaches the grad

course in cosmology at UCLA and is a director of WMAP and he has a feature of his site for cosmology gossip called "news of the universe"

http://www.astro.ucla.edu/~wright/cosmolog.htm

[Martin]

Rats! not a word

it looks like he hasnt updated for a while either

 

well, basically I lack ammunition to convince you that there is something here----there is not yet enough recognition among working cosmologist of the interest of this alternative approach

 

so i have to retreat and wait awhile.

 

anybody who wants to can read about it in the online lectures from the Quantum Gravity symposium this year in Poland

 

http://ws2004.ift.uni.wroc.pl/html.html

 

just go to the main page,

click on "lectures" and find Smolin's third talk on the menu

and click on that. It shows the galaxy rotation curves and the good fit.

but until this gets a little more publicity nobody but a few specialists

is going to want to take the trouble.

[Alexa]

Hi Martin,

 

No, I'm not sophisticated at all. I've just had this conversation already on another forum. We weren't able to go far through it, as we were only two members interested on the subject. So, we dropped it.

 

I agree the DM and DE are related.

 

I thought LAMBDA means Legacy Archive for Microwave Background Data Analysis.

 

Anyway, if you are able to find more about the DE, keep me posted.

 

Thanks,

 

Alexa