Dark matter - dark energy

In summary: There is still much debate over what this "something" is, but it is believed to be a form of energy similar to mass, but with zero physical properties.In summary, Pete and Marlon discuss the difference between dark matter and dark energy, dark energy being the more mysterious of the two. Pete also points out that we are made of baryonic matter, which is made of atoms with protons in them. They go on to discuss how scalar fields are related to the discovery of dark energy and its varying levels of support.
  • #1
marlon
3,792
11
dark matter --- dark energy

Can anyone explain to me the difference between dark matter and dark energy ?

What are their proper relations to the expanding universe?

Dark matter should be taken into account because we know not enough matter that can be held responsible for keeping galaxies together right ?


tanti auguri
marlon
 
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  • #2
marlon said:
Can anyone explain to me the difference between dark matter and dark energy ?

Dark matter is that matter which accounts for the gravitational mass in galazies which makes the rotation curve (rotational speed plotted against distance from center of galaxy) approximately flat. Dark energy is that matter in the universe which is responsible for the accelerating expansion of the universe.

Pete
 
  • #3
Dark matter is considered to be made from exotic particles like WIMPS (WEAKLY INTERACTING MASSIVE PARTICLES) or MACHOS or it could be made from other undiscovered exotic particles.Dark matter might account for the extra gravity that allows stars to have higher velocities than normal in spiral galaxies and that allows galaxies to orbit one another at speeds far higher than Newton's laws allow (Modified Newtonian Dynamics, MOND, a theory by Mordechai Milgrom,changes Newton's force law and accounts for experimental data very closely indeed but no-one knows why MOND works!).Dark energy is considered exist everywhere in the universe and to be more energy like than mass like - hence the name dark energy .Many theorists believe dark energy is a scalar field called quintessence (there is also a version called spintessence).Dark energy has a constant energy density throughout the universe
and the total amount of dark energy increases as space expands (some people think the extra dark energy is made from gravitational potential energy).Dark energy appears as the cosmological constant in Friedmanns's equations and has negative pressure - it is gravitationally repulsive and it is causing the expansion of the universe to accelerate .Observations of the brightness of the same supernovae at different times have confirmed that the expansion is accelerating-the supernovae are dimmer than expected after a fixed time has elapsed, so they are further away than can be explained by a uniform velocity.It is not known for sure that dark matter exists but the
evidence for dark energy is convincing.Dark energy may be dark matter in some theories.Most of the mass of the universe is considered to be in the form of dark energy - at least 70 per cent.
 
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  • #4
pmb_phy said:
Dark matter is that matter which accounts for the gravitational mass in galazies which makes the rotation curve (rotational speed plotted against distance from center of galaxy) approximately flat. Dark energy is that matter in the universe which is responsible for the accelerating expansion of the universe.

Pete


Thanks, Pete for replying.

I have one question though. Do we have any idea on much percent of the "total mass" is dark energy, dark matter and just the kown matter?

for example : 75% dark energy, 14% dark matter, 1% kown matter?

How can we distinguish between dark energy and dark matter ?? How were they discovered ?
 
  • #5
I heard that the universe is something like 70%dark matter. And to answer your last question, dark matter was first proposed because stars around gallaxies accelerate at the smae rate, no matter if they are close or near the black hole in the centre, this is why there is a lot of dark matter found at the edge of gallaxies.
 
  • #6
70 % dark Energy,25 % dark Matter, and 5 % baryonic matter.
We are made from baryonic matter.
.
 
  • #7
But how is this socalled quintessence been discovered and why is the dark energy represented by a scalar field ?

I know that scalar field do not interact through elektromagnetic interactions because of the spin 0 quanta that they carry. Does this have anything to do with the discovery of dark energy.


Nenad, thanks for explaining the dark matter-discovery, i got the point...

Thanks also to my dear Yang-Mills-friend Kurious

marlon (not brando, unfortunately)
 
  • #8
Dark energy was discovered because of the supernovae being dimmer than expected - in 1998, I think.
 
  • #9
marlon: quintessence has not been discovered yet, it's only hypothetical

there are other proposed candidates to act like dark energy: acceleressence, phantom energy, Chaplygin gas, kinessence,...
 
  • #10
kurious said:
Dark energy was discovered because of the supernovae being dimmer than expected - in 1998, I think.


ok, but how is this related exactly to dark energy and what about that scalar field quintessence ?

regards
marlon
 
  • #11
Marlon:
ok, but how is this related exactly to dark energy and what about that scalar field quintessence ?

Kurious:

The brightness of light from a supernova at one point in time can be compared to the brightness of light from the same supernova at a later time.The light is much less bright than expected when it is measured at the later time because something is causing the supernova to move away from the Earth faster than expected.
Dark energy is believed to be causing this faster than expected movement by speeding up the rate of expansion of space.Einstein had already guessed that such a negative pressure - which is what dark energy is - could exist, and put it into one of his equations as the so-called cosmological constant.Nobody knows what dark energy is made from- is it waves particles etc.Some theorists are saying that dark energy is
the energy associated with a scalar field called quintessence - a field new to science -
that fills all of space.
 
  • #12
thanks to you all for explaining

marlon
 

1. What is dark matter?

Dark matter is a type of matter that does not interact with light or other forms of electromagnetic radiation. It cannot be seen or detected using traditional methods, but it is believed to make up about 85% of the total matter in the universe. Its presence is inferred through its gravitational effects on visible matter.

2. How is dark matter different from regular matter?

Dark matter is different from regular matter in that it does not emit or absorb light or other forms of electromagnetic radiation. It also does not interact with normal matter through the strong or weak nuclear forces, only through gravity. This makes it very difficult to detect and study.

3. What is dark energy?

Dark energy is a theoretical form of energy that is believed to make up about 70% of the total energy in the universe. It is thought to be responsible for the observed accelerated expansion of the universe. Its exact nature is still unknown, but it is believed to have a repulsive effect on matter and space.

4. How is dark energy related to dark matter?

Dark energy and dark matter are two separate and distinct concepts. While dark matter is believed to make up the majority of the total matter in the universe, dark energy is thought to make up the majority of the total energy. They are both still poorly understood and are the subject of ongoing research and study in the field of astrophysics.

5. What are some current theories about the nature of dark matter and dark energy?

There are several theories about the nature of dark matter and dark energy, but none have been definitively proven. Some current theories suggest that dark matter is made up of weakly interacting massive particles (WIMPs) or axions, while others propose modifications to the laws of gravity to explain the observed effects. As for dark energy, some theories suggest it is a type of energy field that permeates space, while others propose additional dimensions or a breakdown of Einstein's theory of general relativity.

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