Why do we think there is dark matter and dark energy?

[Anthony Morris]

I am trying to figure out how these forms of matter are supposed to work but I can't even figure out why we know about them or think we do. I do understand that galaxies appear to be moving away faster than expected but I don't understand why that's relevant. Any good reading on the matter (ahem) that a lay-person could understand?

[Janus]

First off, dark matter and dark energy are two different things and the reasons for expecting their existence are completely unrelated. The only thing they have in common is that they have "dark" in their names.

The initial evidence for dark matter came from observations of how stars move in galaxies.    Galaxies are formed from stars that are mutually orbiting each other.  If we look at a galaxy, and estimate its mass by the matter we can see, we find that there does not appear to be enough to hold the galaxy together. At the speed the stars are orbiting, they should fly apart.  We also know how these stars should orbit if the mass is contained to the shape we see it as having.   Not only does the galaxy have more mass than that we can see, but the unseen mass must be distributed a lot differently than the part we do not see.  For example, in a typical spiral galaxy, a good deal of the mass must be located above and below the disk-like shape we see.  If it was made of normal matter, we should see it, if not in the visible spectrum, it should be visible at some other spectrum.  This leads us to believe that whatever is causing that extra mass is not made of normal matter, but a type of matter that does not emit or interact with light or any part of the electromagnetic spectrum.  Thus the term "dark matter.   There have been attempts to explain the discrepancy through developing different models for how gravity behaves, but to date, none have been consistent with all the observations we have made.

Dark energy concerns itself with the expansion of the universe.  We have known for a long time that the universe is expanding and that distant galaxies are moving away from us.  But until couple of decades ago, we assumed that the mutual gravity between the different parts of the universe was slowing this expansion down over time. What we did not know was whether this was enough to eventual stop the expansion all together.  In the 1990's a study was made to try to determine if this was the case or not.  Basically it worked because as we look at distant galaxies, we are seeing them as they were when the light left them.  Thus as we look further away we are looking further into the past.   Thus, to explain it simply, by comparing various galaxies' distances to how fast they appear to be receding from us,  you can work out how the expansion of the universe has change over time. The surprise came when it was discovered that the universe's expansion was not slowing down, but was speeding up.  Not only was it mutual gravity not enough to stop its expansion, but something was overcoming the gravity and pushing the universe  apart.   They decided to call this unknown influence "dark energy" (mainly because they had already coined the term "dark matter") . We really know very little about dark energy, and the term really just is a place holder for whatever it turns out to be. (Much in the way the terms "X-rays" was coined before we learned that they were just a certain part of the electromagnetic spectrum.)

 

[Anthony Morris]

Thank you Janus. I appreciate it.

[T. McGrath]

16 hours ago, Anthony Morris said:

I am trying to figure out how these forms of matter are supposed to work but I can't even figure out why we know about them or think we do. I do understand that galaxies appear to be moving away faster than expected but I don't understand why that's relevant. Any good reading on the matter (ahem) that a lay-person could understand?

First and foremost you need to understand that both "Dark Matter" and "Dark Energy" are merely placeholders, or labels, for something that we do not understand.  It does not mean that either are "dark."  They are only "dark" because we do not understand either.  So they are really a description of our ignorance than anything real.  As Janus correctly pointed out, Dark Matter and Dark Energy are completely unrelated.

Dark Matter's history goes back to the 1930s.  Fritz Zwicky first discovered the unusual rotational periods while working on galaxy clusters.  It went largely unnoticed until the 1970s when Vera Rubin began studying galactic rotation.  For some unexplained reason, galaxies were rotating far faster than they should be for the amount of visible matter that they have.  Galaxies were rotating as if they had considerably more mass, mass that did not react to light.  Hence the term "Dark Matter" was created to label this invisible matter.  Since the late 1970s there has been a search for this missing matter.  We can identify the effects Dark Matter has, such as the rotation of the galaxies, and also gravitational effects that causes light to bend, but we are no closer to identifying what Dark Matter actually is.  There have been lots of theories, from Massive Compact Halo Objects (MACHOS) to Weakly Interacting Massive Particles (WIMPS), but so far none of them have panned out.

In the case of Dark Matter we have something tangible and observable that we have not yet been able to explain.  We have even made maps of Dark Matter using the light as it bends passing through such a strong gravitational field.  We just can't see it, or explain what it is.  Dark Energy, on the other hand, is something completely different.

The term "Dark Energy" came along in the 1990s in an attempt to explain why the universe was not just expanding, but accelerating.  Vesto Slipher first discovered galactic red-shifts in 1912, but it took Edwin Hubble to correlate these red-shifts into distance.  While not a very accurate means of determining distance, it was sufficient to identify that galaxies were moving away from each other, and the further away they were the faster they appeared to be moving.  In 1927 Georges Lemaitre, using Einstein's 1915 Theory of General Relativity, came up with his "Primordial Atom", which would be latter derisively referred to as the "Big Bang" by its biggest critic, Fred Hoyle.  This debate on whether the universe was static and everlasting or had a beginning continued until 1964.  When the cosmic background radiation was detected in 1964 it was the crucial evidence needed to sway the majority of astronomers and astrophysicists to accept the "Big Bang."

Numerous attempts have been made to determine the age of the universe, beginning with Edwin Hubble.  Unfortunately Hubble's initial estimate at the rate the universe was expanding (also known as the "Hubble Constant" today) was 500 km/s/Mpc or about 153 km per second per million light-years, which gave him an estimate of ~2 billion years.  Hubble's mistake was his use of Cepheid variables as a "Standard Candle."

We have since substituted Type Ia Supernovae (SN) to be our new "Standard Candle" and the current age of the universe (13.813± 0.038 billion years), making the Hubble Constant  67.31± 0.96 km/s/Mpc or ~20.65 km per second per million light-years.  This is also where "Dark Energy" first makes its appearance.  During the 1990s a concerted effort was made to record as many Type Ia SN as possible, and when the distances to these Type Ia SN were measured they concluded that the universe is not just expanding (as Hubble noted), but accelerating.  Unable to explain this acceleration they attributed it to some unseen, and before now, undetected form of energy which they labeled "Dark Energy."

Since that label was created we have since discovered that Type Ia SN are not the "Standard Candle" astronomers originally presumed them to be.  Originally it was assumed that all Type Ia SN had an absolute magnitude of -19.46.  However, in 2006 we discovered the first of several "superluminous" Type Ia SN.  Then in 2013 a whole new classification of supernovae was created.  This new classification, Type Iax SN, had an absolute magnitude ranging between -14.2 and -18.9, much dimmer than Type Ia SN.  Furthermore, it is estimated that between 18% and 48% of all the Type Ia SN prior to 2013 have been misclassified and should actually be Type Iax SN.  Which calls into question the Type Ia SN data collected during the 1990s used to calculate the age of the universe and its alleged acceleration.

Exactly like Hubble's mistake using Cepheid variables as his "Standard Candle" in 1927 to determine the age of the universe, our understanding of "Dark Energy" could be attributed to the Type Ia SN we also mistakenly thought was a "Standard Candle" in the 1990s.  We can distinguish the difference between Type Ia SN and Type Iax SN with sufficient data, but identifying "superluminous" Type Ia SN and distinguishing them from normal Type Ia SN is another matter entirely.  Which means that there is no such thing as a "Standard Candle" in astronomy.

 

Sources:
A History of Dark Matter - arXiv : 1605.04909v2, May 24, 2016
WIMPs & MACHOs - Encyclopedia of Astronomy & Astrophysics, 2002 [PDF]
A Brief History of Dark Energy - Astrophysics & Space Science, Volume 319, Issue 1, January 2009 (free preprint)
What is the Hubble Constant? - Space.com, March 21, 2014
Type Iax Supernovae: A New Class of Stellar Explosion - The Astronomical Journal, Volume 767, Number 1, March 25, 2013
A Comparative Study of the Absolute Magnitude Distributions of Supernovae - The Astronomical Journal, Volume 123, Number 2, 2002

Edited October 12, 2017 by T. McGrath

[Anthony Morris]

Thanks for all the info. I feel better about not knowing any of this before. And thanks for the links too. I'm looking forward to reading them.

[T. McGrath]

On 10/12/2017 at 0:10 PM, T. McGrath said:

Type Iax Supernovae: A New Class of Stellar Explosion - The Astronomical Journal, Volume 767, Number 1, March 25, 2013

The link above is bogus.  I must have accidentally posted the wrong URL.  Below is the correct link.  It is a free article.

Type Iax Supernovae: A New Class of Stellar Explosion - The Astronomical Journal, Volume 767, Number 1, March 25, 2013

[Airbrush]

On ‎10‎/‎13‎/‎2017 at 1:04 PM, Anthony Morris said:

Thanks for all the info. I feel better about not knowing any of this before. And thanks for the links too. I'm looking forward to reading them.

Have you ever looked up anything on Wikipedia.org?  Just enter "dark matter" and it will give you an overview of dark matter.  Enter "dark energy" and it will tell you something about that.

[Anthony Morris]

On ‎10‎/‎15‎/‎2017 at 0:01 PM, Airbrush said:

Have you ever looked up anything on Wikipedia.org?  Just enter "dark matter" and it will give you an overview of dark matter.  Enter "dark energy" and it will tell you something about that.

You know, it's funny, but I never really do look up anything in Wikipedia unless it appears on a search I'm doing in Bing or Google. 

[Roamer]

On 12-10-2017 at 7:47 AM, Janus said:

thus, to explain it simply, by comparing various galaxies' distances to how fast they appear to be receding from us,  you can work out how the expansion of the universe has change over time.

Has the speed of the light we observe been measured to be still at the maximum lightspeed. (aka didn't it slow down after millions of years of travel)

I couldn't find it in a quick google-search, although there was an article suggesting light can be slowed through changing it's pattern.

[Janus]

1 hour ago, Roamer said:

Has the speed of the light we observe been measured to be still at the maximum lightspeed. (aka didn't it slow down after millions of years of travel)

I couldn't find it in a quick google-search, although there was an article suggesting light can be slowed through changing it's pattern.

If you are suggesting that light coming from some distant galaxy is traveling slower when it reaches the Earth than light produced locally, then this would produce noticeable effects. One would be in stellar aberration.  When look at a star that is located on a line which is at a right angle to the Earth's orbital motion.  It apparent position shifts a bit which is determined by the relationship between the speed of light and Earth's orbital speed.   Since the direction of the Earth's motion changes over the course of a year, the direction of the apparent shift also changes.   

The point is, that if the speed of light arriving from a further galaxy was slower upon reaching Earth that light from a closer galaxy, we would measure a larger apparent shift in the position of the further galaxy than we would for the closer galaxy.  This would be noticeable in any Hubble picture which contained galaxies at varying distances.  

When the Hubble takes a picture of some distant part of the universe, it can't just take a quick snap shot, it needs a very long exposure. Much longer than the scope can remain pointed at a single point of the Sky as it orbits the Earth.  So what is done is that the Telescope points at a part of  the sky for a while, records what it sees, and then waits until it's back into position again to continue the image, and repeats this process over and over until the total exposure time is met.    A single image can takes months to complete.  But this means that the apparent shift due to aberration also changes while the image if being formed.  This is fine as long as the speed of the light arriving from all those galaxies is the same, as the shift will be the same and can  be accounted for.  But if light from further galaxies was slower when it reached Earth, those galaxies would exhibit a larger shift than nearer ones.  The image compiled of multiple exposures taken over a long period of time would show "smearing" of these further galaxies which increased the further the galaxy was away.

[T. McGrath]

17 hours ago, Roamer said:

Has the speed of the light we observe been measured to be still at the maximum lightspeed. (aka didn't it slow down after millions of years of travel)

I couldn't find it in a quick google-search, although there was an article suggesting light can be slowed through changing it's pattern.

Think of the speed of light as the current maximum speed limit for anything with mass.  That does not mean that all photons are traveling at light speed, only that anything with mass cannot travel faster than light speed.  If photons have to pass through a medium other than a vacuum then you can expect that they will not be traveling at their maximum speed.

At Harvard University they conducted an experiment using a Bose--Einstein condensate as the medium to slow the photons.

Having said that, it has been suggested that in the early universe the maximum speed of light was actually faster than 299,792,458 m/s and has slowed to its current maximum speed.

Sources:
Spatially Structured Photons that Travel in Free Space Slower than the Speed of Light -  Science, Volume 347, Issue 6224, Pages 857-860, February 2015 (free preprint)
Critical Geometry of a Thermal Big Bang - Physical Review D,  Volume 94, Issue 10, November 2016 (free preprint)

[swansont]

On 10/12/2017 at 4:10 PM, T. McGrath said:

First and foremost you need to understand that both "Dark Matter" and "Dark Energy" are merely placeholders, or labels, for something that we do not understand.  It does not mean that either are "dark."  They are only "dark" because we do not understand either. 

Well, no. It literally means dark matter is dark, as as it does not interact via electromagnetism (i.e. no photons), which is one thing we do understand about it. Its interaction is gravitational.

11 hours ago, T. McGrath said:

Think of the speed of light as the current maximum speed limit for anything with mass.  That does not mean that all photons are traveling at light speed, only that anything with mass cannot travel faster than light speed. 

Light might not travel at c, but photons do.

[Silvestru]

T. McGrath 

Quote

Think of the speed of light as the current maximum speed limit for anything with mass

Massive objects would not be able to travel at c.

Edited November 2, 2017 by Silvestru
wrong quote

[swansont]

9 minutes ago, Silvestru said:

T. McGrath 

Massive objects would not be able to travel at c.

Which makes it the limiting value, in the math sense of the word "limit"

[Silvestru]

My understanding of the statement was that objects with mass cannot travel faster than the speed of light. If the speed limit on a road is 80mph you can legally travel at 80 but not go over. What I said is that it's impossible to travel at 80. Sorry If I misunderstood.

[swansont]

11 minutes ago, Silvestru said:

My understanding of the statement was that objects with mass cannot travel faster than the speed of light. If the speed limit on a road is 80mph you can legally travel at 80 but not go over. What I said is that it's impossible to travel at 80. Sorry If I misunderstood.

It's an easy mistake to make, since the everyday use of the word differs from the technical. Happens a lot in physics.

[Much Ado]

Is mass the only "creator" of gravity?

Assumptions = Limitations 

[Strange]

28 minutes ago, Much Ado said:

Is mass the only "creator" of gravity?

NO. Energy, pressure, momentum flux, sheet stress and various other things also contribute. Described by the stress-energy tensor: https://en.wikipedia.org/wiki/Stress–energy_tensor

It doesn't even mention mass - that is included under energy.

Quote

Assumptions = Limitations 

Indeed. Which is why scientists always try and identify any assumptions and test them.

Edited November 2, 2017 by Strange

[Much Ado]

Thank you, please then, why is "Dark Matter" inferred in galaxies relative to mass - gravity deficiencies ? 

[zapatos]

On 11/1/2017 at 3:36 AM, T. McGrath said:

Think of the speed of light as the current maximum speed limit for anything with mass.  That does not mean that all photons are traveling at light speed, only that anything with mass cannot travel faster than light speed.  If photons have to pass through a medium other than a vacuum then you can expect that they will not be traveling at their maximum speed.

It also cannot travel AT light speed. Which I think is what Silvestru was getting at.

[Strange]

54 minutes ago, Much Ado said:

Thank you, please then, why is "Dark Matter" inferred in galaxies relative to mass - gravity deficiencies ? 

Did you read the second post from Janus? 

There are multiple lines of evidence for dark matter: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence

[Much Ado]

Very helpful thank you.

To clarify my thoughts, why is the effective condition referred to as dark "matter". The definition of matter is clear enough, or is it? Must it be matter.

 

 

[Strange]

4 minutes ago, Much Ado said:

Very helpful thank you.

To clarify my thoughts, why is the effective condition referred to as dark "matter". The definition of matter is clear enough, or is it? Must it be matter.

Because the simplest explanation for galaxy rotation curves, for example, is to add some extra matter, with a suitable distribution. Of course, other explanations are possible, such as a modification to the way we think gravity works so, for a while "dark matter" was just a placeholder for "whatever it is that causes the effects we observe" ("dark energy" is still such a placeholder).

However, none of the modified gravity theories actually work (*). Plus there are multiple lines of evidence that suggest dark matter is actually some form of matter. But until we have some sort f direct detection, the options will still be open.

(*) Plus, most of the alternative gravity models are ruled out by the results from the recent gravity wave observation: https://medium.com/starts-with-a-bang/merging-neutron-stars-deliver-deathblow-to-dark-matter-and-dark-energy-alternatives-b5e6d2f44d37

[Much Ado]

Great, thanks again. Your links were good.

Do you know of any terminology that refers to existence, other than matter or energy, I am thinking that the effective condition is neither matter nor energy.

 

[Strange]

1 minute ago, Much Ado said:

Great, thanks again. Your links were good.

Do you know of any terminology that refers to existence, other than matter or energy, I am thinking that the effective condition is neither matter nor energy.

Other than modified gravity there are some more far out ideas like: https://en.wikipedia.org/wiki/Chaplygin_gas. But I don't really know how well they fit the evidence.