Hello folks, After a short hiatus, but always reading new commentary from respected astrophysicists and space jounalists, I proudly remain in the camp that Dark Matter is nothing more than gravity behaving differently at long distances and closely resembles the Modified Dynamics (MOND) theory line. The only discrepancy is that the equations for MOND do not emulate gravity exactly. Other forces in the universe also are affecting the appearance of the galaxies by JWST and other telescopes. This holds true in the cases of Bullet Clusters and Gravitational Lensing examples where MOND is getting so much ridicule. The research for the right answers is continuing, but right now, I am holding form that Dark Matter is not a real substance. And for those other believers that Dark Matter can be something from another universe or dimension that pops in and out of our universe, that will forever remain a mythical event.
Dark Matter Revisited
- Thread starter shellyhe
- Start date
I believe that dark matter is what makes up the fabric of spacetime and we won’t find it until we figure out how to look outside of spacetime. If in the early universe the dark matter all condensed at the same time this would have resulted in black holes appearing much larger and gravity wells much deeper. In these wells time would pass more slowly relative to use which would match the slower pulsars and to us faster galaxy developement. Pulsar timing also shows a possible phase transition in the early universe. Over time as the ratio of liquid to gaseous dark matter dropped which reduced the time differential across the universe. Not taking into account the redshift due to climbing out of those gravity wells biases our distance calculations making us think the universe is expanding. It would be interesting to calculate the neutrino mass without the assumption of an expanding universe.
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Trying to understand Dark Matter
I am not an astrophysics but consider myself relatively intelligent. I am finding it very difficult to understand the concept of dark matter.
In simplistic terms, as I understand it, astrophysics currently believe that there is far more mass in the universe than can be explained by the visually observable universe (visible, infrared, x-ray and gamma ray light). The observed effects of gravity in distant galaxies is claimed to show far more rotational speed that would be based on the visually observed mass. Based on these assumptions it is stated that 85% of this calculated mass is something we cannot see or indirectly detect. “Dark Matter” is the placeholder name for this “unknown excess” mass that cannot or has not been observed or explained.
It is also my understanding that there are many things of mass that we expect exist which we cannot visually observe. Rogue moons, planets, failed stars, brown dwarfs, interstellar asteroids are just being found within our Milky-way galaxy. Scientists say there are likely millions of stellar-mass black holes in our galaxy. Multiply that by the billons of galaxies, along with the speculated microscopic blackholes and many other possible not yet know items of mass in the universe. Voyager 1 has detected a faint, monotonous hum from plasma (ionized gas) in interstellar space. Therefore, it could be said that all of space contains plasma which has mass.
All of the above could account for the assumed mass within the current known universe.
I must therefore question the 85% estimated Dark Matter hypothesis.
Would someone please address this issue providing a detailed explanation which a non-astrophysicist/quantum physicist could understand.
I am not an astrophysics but consider myself relatively intelligent. I am finding it very difficult to understand the concept of dark matter.
In simplistic terms, as I understand it, astrophysics currently believe that there is far more mass in the universe than can be explained by the visually observable universe (visible, infrared, x-ray and gamma ray light). The observed effects of gravity in distant galaxies is claimed to show far more rotational speed that would be based on the visually observed mass. Based on these assumptions it is stated that 85% of this calculated mass is something we cannot see or indirectly detect. “Dark Matter” is the placeholder name for this “unknown excess” mass that cannot or has not been observed or explained.
It is also my understanding that there are many things of mass that we expect exist which we cannot visually observe. Rogue moons, planets, failed stars, brown dwarfs, interstellar asteroids are just being found within our Milky-way galaxy. Scientists say there are likely millions of stellar-mass black holes in our galaxy. Multiply that by the billons of galaxies, along with the speculated microscopic blackholes and many other possible not yet know items of mass in the universe. Voyager 1 has detected a faint, monotonous hum from plasma (ionized gas) in interstellar space. Therefore, it could be said that all of space contains plasma which has mass.
All of the above could account for the assumed mass within the current known universe.
I must therefore question the 85% estimated Dark Matter hypothesis.
Would someone please address this issue providing a detailed explanation which a non-astrophysicist/quantum physicist could understand.
Jeff, you have a basic misunderstanding here, we can account for around 15% of all energy in the observable Universe, this includes all baryonic matter as mass and energy are interchangeable.
The 85% you mentioned is not all Dark Matter - take a look at this graphic.
Now, as you can see, in this graphic, Dark Matter only makes up around 26% of the matter in the Observable Universe, some 63% is believed to be Dark Energy, however, this assumptions has been called into question lately due to the publication in The Monthly Notices of the Royal Astronomical Society, of a paper by Antonia Seifert, Zachary G Lane, Marco Galoppo, Ryan Ridden-Harper, David L Wiltshire called Supernovae evidence for foundational change to cosmological models.
The new evidence supports the "timescape" model of cosmic expansion, which doesn't have a need for dark energy because the differences in stretching light aren't the result of an accelerating Universe but instead a consequence of how we calibrate time and distance. It takes into account that gravity slows time, so an ideal clock in empty space ticks faster than inside a galaxy.
The model suggests that a clock in the Milky Way would be about 35 per cent slower than the same one at an average position in large cosmic voids, meaning billions more years would have passed in voids. This would in turn allow more expansion of space, making it seem like the expansion is getting faster when such vast empty voids grow to dominate the Universe.
With regards Dark Matter, despite being invisible, its presence is inferred through its gravitational effects on visible matter, radiation, and the large-scale structure of the Universe.
So what is dark matter?
Dark Matter refers to a form of matter that does not emit, absorb, or reflect light, making it undetectable through electromagnetic radiation. It interacts primarily through gravity, and its existence is postulated to explain several astronomical observations that cannot be accounted for by visible matter alone.
How Do We Know Dark Matter Exists?
1. Galaxy Rotation Curves:
- Observations of spiral galaxies show that stars at their edges orbit faster than predicted by the visible mass (stars, gas, and dust). This discrepancy implies the presence of unseen mass extending beyond the visible galaxy.
- Massive objects, such as galaxy clusters, bend light from background objects. The amount of bending often exceeds what can be attributed to visible matter, suggesting the presence of additional invisible mass.
- The CMB, a relic of the Big Bang, shows patterns that can only be explained if there is much more matter than we can see, with Dark Matter being a significant contributor.
- The distribution and formation of galaxies and galaxy clusters over time require a gravitational framework provided by Dark Matter to clump and form structures.
- Measurements of galaxy cluster masses via the motion of galaxies, hot gas observed in X-rays, and gravitational lensing reveal far more mass than visible matter accounts for.
Dark Matter affects the Universe in several profound ways:
- Gravitational Binding - It provides the gravitational "scaffolding" that holds galaxies and galaxy clusters together. Without it, galaxies would disperse.
- Formation of Structures - During the early Universe, Dark Matter clumped under gravity, forming the seeds for galaxies and larger structures.
- Motion of Stars and Gas - It influences the dynamics within galaxies, explaining their flat rotation curves.
- Cosmological Evolution - It impacts the expansion rate and structure of the Universe, interacting indirectly with normal matter through gravity.
The nature of Dark Matter remains unknown, but several hypotheses exist:
1. WIMPs (Weakly Interacting Massive Particles)
- Hypothetical particles that interact only via gravity and weak nuclear forces. They are a popular candidate and are sought through particle physics experiments.
- Ultralight particles predicted by extensions of quantum field theory. They might form a diffuse field that permeates the Universe.
- Hypothetical heavier versions of neutrinos that do not interact via normal weak nuclear forces, only through gravity.
- These include objects like black holes, neutron stars, or brown dwarfs. However, they are not sufficient to explain the observed effects.
- Some theories suggest modifying our understanding of gravity (e.g., MOND—Modified Newtonian Dynamics) to explain the observations without invoking Dark Matter. However, these fail to account for all evidence. This has been largely disproved in recent months with one of its biggest proponents now actively speaking out against the theory. I have included it for completeness.
1. Direct Detection
- Experiments aim to observe rare interactions between Dark Matter particles and normal matter. Facilities like LUX-ZEPLIN (LZ) and XENON use ultra-sensitive detectors to look for WIMP interactions.
- Dark Matter annihilations or decays might produce high-energy gamma rays, neutrinos, or other particles. Observatories like the Fermi Gamma-ray Space Telescope search for these signals.
- High-energy particle colliders, like the Large Hadron Collider (LHC), may produce Dark Matter particles that escape detection, inferred through missing energy in particle collisions.
- Improved telescopes and gravitational wave detectors might provide additional indirect evidence by studying the effects of Dark Matter on cosmic structures.
- Experiments like ADMX (Axion Dark Matter Experiment) use resonant cavities to search for the conversion of axions into photons in a magnetic field.
In essence, Dark Matter remains one of the Universe's greatest mysteries, with its discovery promising to revolutionize our understanding of physics and cosmology.
I hope this helps you get a better understanding of Dark Matter and do not confuse it with Dark Energy.
I like your answer but just don't think that the objects mentioned is enough to keep any particular galaxy from falling apart. For example, if there were many black holes near a large visible galaxy, there would be some visible evidence. Maybe it could be a combination of unseen matter, as you mentioned, plus the MOND gravity adjustment and some other unseen forces in the universe. Just no pure dark matter.
01-11-25
Jim, I greatly appreciate the information in your response.
I found your reference to the "timescape" model very intriguing. It provides an explanation that does not rely on inventing new/unknowns to account for observable interactions.
Concerning the “Dark Matter,” I’m still not sure why the items I postulated could not account for at least the matter portion of the discussion.
You provided a graphic showing that Dark Matter makes up around 26% of the matter in the observable Universe. For sack of this discussion, I will accept that number. Hopefully we can address Dark Energy issue in the future.
For now, however, in your explanation “How Do We Know Dark Matter Exists?” you at least partly supported my postulation.
“Galaxy Rotation Curves … stars at their edges orbit faster than predicted by the visible mass … This discrepancy implies the presence of unseen mass extending beyond the visible galaxy.
Gravitational Lensing … Massive objects … bend light from background objects. The amount of bending often exceeds what can be attributed to visible matter, suggesting the presence of additional invisible mass.
Cosmic Microwave Background (CMB) Radiation … The CMB … shows patterns that can only be explained if there is much more matter than we can see …
Galaxy Clusters and Missing Mass … Measurements of galaxy cluster masses via the motion of galaxies, hot gas observed in X-rays, and gravitational lensing reveal far more mass than visible matter accounts for.
Effects of Dark Matter on Normal Matter … It impacts the expansion rate and structure of the Universe, interacting indirectly with normal matter through gravity.”
Why is it necessary to say Dark Matter is not normal matter. Most of your explanation implies that what the term Dark Matter is used to describe what is simply matter which as of yet is not visibly observable.
I again ask the question why do we need “Dark Matter”? It still appears that the effects of Dark Matter could be explained by normal matter that just has not yet been visually observed.
Jim, I greatly appreciate the information in your response.
I found your reference to the "timescape" model very intriguing. It provides an explanation that does not rely on inventing new/unknowns to account for observable interactions.
Concerning the “Dark Matter,” I’m still not sure why the items I postulated could not account for at least the matter portion of the discussion.
You provided a graphic showing that Dark Matter makes up around 26% of the matter in the observable Universe. For sack of this discussion, I will accept that number. Hopefully we can address Dark Energy issue in the future.
For now, however, in your explanation “How Do We Know Dark Matter Exists?” you at least partly supported my postulation.
“Galaxy Rotation Curves … stars at their edges orbit faster than predicted by the visible mass … This discrepancy implies the presence of unseen mass extending beyond the visible galaxy.
Gravitational Lensing … Massive objects … bend light from background objects. The amount of bending often exceeds what can be attributed to visible matter, suggesting the presence of additional invisible mass.
Cosmic Microwave Background (CMB) Radiation … The CMB … shows patterns that can only be explained if there is much more matter than we can see …
Galaxy Clusters and Missing Mass … Measurements of galaxy cluster masses via the motion of galaxies, hot gas observed in X-rays, and gravitational lensing reveal far more mass than visible matter accounts for.
Effects of Dark Matter on Normal Matter … It impacts the expansion rate and structure of the Universe, interacting indirectly with normal matter through gravity.”
Why is it necessary to say Dark Matter is not normal matter. Most of your explanation implies that what the term Dark Matter is used to describe what is simply matter which as of yet is not visibly observable.
I again ask the question why do we need “Dark Matter”? It still appears that the effects of Dark Matter could be explained by normal matter that just has not yet been visually observed.
Glad to be useful.
It is an intriguing concept and theory, I have read the paper and run the numbers, it checks out - but that may not make it true or even all of the answer, nature has a habit of combining "solutions" to create confusion until we realise we need to take off the blinkers and join ideas together, just like Eintein did to create The Special Theory of Relativity.
It is believed that that the Universe is critically balanced between being open and closed. We derive this fact from the observation of the large scale structure of the Universe. It requires a certain amount of matter to accomplish this result. Call it M.
We can estimate the total baryonic matter of the universe by studying Big Bang nucleosynthesis. This is done by connecting the observed He/H ratio of the Universe today to the amount of baryonic matter present during the early hot phase when most of the helium was produced.
Once the temperature of the Universe dropped below the neutron-proton mass difference, neutrons began decaying into protons. If the early baryon density was low, then it was hard for a proton to find a neutron with which to make helium before too many of the neutrons decayed away to account for the amount of helium we see today. So by measuring the He/H ratio today, we can estimate the necessary baryon density shortly after the Big Bang, and, consequently, the total number of baryons today.
It turns out that you need about 0.05 M total baryonic matter to account for the known ratio of light isotopes. So only 1/20 of the total mass of the Universe is baryonic matter.
It is up to you to either accept or dimiss the 26%, but it is a science fact based on current knowledge. This may change in the future, it may reduce or increase, personally I think there are flaws in the methodology I described above, but we must start somewhere, and we can only go on the information we have.
I did not support your postulation about planets etc because they are baryonic matter that is accounted for in the 26% of the Universe. However, Yes, we know, or the evidence strongly suggests that there is some form of matter that interacts only by gravity and we see the vidence for this across the universe - except for some galaxies that appear to be DM poor!
That is correct, that is the primary way we know there is material out there that interacts via gravity but in all other respects is "dark" to our current technology. But of you think about it, IR was once "dark" because although we could feel it and see its effects, we had no idea what it was until we developed technology that could capture IR radiation - the same is true for all parts of the electromagnetic spectrum outside of visible light - afterall, UV lamps are still caled "black lamps". It may be dark now, but once discovered properly it will not longer be dark.
Light from distant objects has been seen to divert or refract in ways that cannot be accounted for by the observable mass, but when the motion of the stars on the outer edge of the galaxy are observed the mass of unseen material is calculated, the gravitational force of this is added and the gravitational lensing is accounted for - but not in all cases.
Fluctuations in the cosmic microwave background (CMB) are caused by a number of factors, including:
- Inflation
A brief period of rapid expansion in the universe a fraction of a second after the Big Bang. This caused points in space that were far apart to become neighbors, resulting in similar temperatures in the CMB. LINK
- Density fluctuations
The CMB's temperature fluctuations are thought to be a reflection of density fluctuations in the early universe. Photons that were in denser regions of space lost energy to gravity, making them colder than average. LINK
- Interaction with matter
Some CMB photons interact with matter on their way from the surface of last scattering to the rest of the universe. This is known as the Sunyaev-Zeldovich effect. It is not connected to Dark Matter.
LINK - Dust and gas in the Milky Way
These are known as foregrounds, and are a source of tertiary fluctuations. This is covered in the link above from Cambridge University.
It has been observed in clusters of galaxies that the motion of galaxies within a cluster suggests that they are bound by a total gravitational force due to about 5-10 times as much matter as can be accounted for from luminous matter in said galaxies. And within an individual galaxy, you can measure the rate of rotation of the stars about the galactic center of rotation. The resultant "rotation curve" is simply related to the distribution of matter in the galaxy. The outer stars in galaxies seem to rotate too fast for the amount of matter that we see in the galaxy. Again, we need about 5 times more matter than we can see via electromagnetic radiation. These results can be explained by assuming that there is a "dark matter halo" surrounding every galaxy.
Yes, that is not in dispute.
Dark Matter, as I said above and have said in other posts, is simply material we are unable to detect, therefore it is dark to our current technology, thus is is not Baryonic matter and thus not normal matter as we know it. That is not to say it is something super exotic, but just material, likely of various types, that we cannot see. For all we know, beings who live on planets in the dark matter universe may see out baryonic Universe as the "dark" or missing matter - unlikely, but nothing is impossible until conclusively rulled out.
We need Dark Matter to explain the motion of stars on the outer fringes of galaxies that orbit too fast based on both Newtonian and Einsteinian gravity, even MOND has been ruled out now. Further, the motions of whole galaxies in clusters cannot be accounted for the detectable mass we can see and account for. Galaxy clusters cause lensing that exceeds what their visible/detectable mass can account for, and this gets worse the larger the galaxy cluster, so it is clear there is something in the Universe that appears to only interact via gravity and we cannot otherwise detect - so we need Dark Matter to explain observations or the entire standard model and Relativity is flawed - and thousands of experiments have proved both to be correct in many ways although it is accepted that there are flaws in all our major theories.
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