Cosmology

Dark Energy and Dark Matter: A Quick Introduction

Dark matter has been hypothesized since the 1930s. Many have heard that a female astronomer Vera Rubin and her co-authors, especially Holland Ford, were responsible for discovering dark matter almost 50 years ago.
This is not correct. Dark matter was first hypothesized and observed by Fritz Zwicky, a Swiss astronomer working at CalTech, in the 1930s. He made observations of the Coma cluster of galaxies that showed that the velocities of galaxies within the cluster, relative to its center, were 3 times higher than expected. This meant the kinetic energy was around 10 times higher and that the gravitational mass of the cluster should be an order of magnitude higher than previously thought.
Zwicky assumed there was much more mass that was not in stars or gas, and not visible, and called it dark matter.
In the 1970s, Rubin, Ford and other astronomers confirmed an unexpected gravitational effect at the edges of many galaxies. In this case one looks at the rotation rate of the galaxy in its outer regions, measuring either stellar velocities or the velocities of gas clouds. Consistently one finds that the galaxies are all rotating faster than expected (the rotation curves of velocity versus distance flatten out, rather than drop off). Most of the matter appears to be in the outer regions, in the galactic halos as they are called.
The effects of dark matter are even seen in the cosmic microwave background observations. The radiation was released at a time when the universe was only 380,000 years old, and the spectrum of density fluctuations reflects the presumed effects of dark matter.
Hot gas detected by Chandra in X-rays. Other telescopes were used to detect the bulk of the matter in the clusters, which turns out to be dark matter (highlighted in blue). Original Image
Credit: X-ray: NASA/
Now all such observations of dark matter are at the scale of galaxies, groups and clusters of galaxies, and the universe as a whole. Our own solar system and the central regions of our galaxy and other galaxies have negligible amounts of dark matter. The measurements at the various large scales, however, are consistent with about 5 times as much dark matter as ordinary matter in the universe.
Yet, these are all indirect measurements. What is dark matter, really? Various ordinary matter possibilities such as brown dwarf stars, white dwarf stars, and black holes have been ruled out. The most popular candidates are exotic particles such as WIMPs (massive supersymmetric particles), axions (very light masses), or sterile (heavier) neutrinos.
But all of these particle options are theorized entities that have never been observed directly. In fact there are many Earth-based and space-based experiments that have attempted to directly detect the various proposed types of dark matter. On Earth, these experiments are placed in deep mines to avoid contamination from cosmic rays and other sources. To date, none of these experiments has succeeded; collectively they have only been placing tighter and tighter constraints on the possible masses and cross-sections for interaction of dark matter candidates with ordinary matter.
At the very least, we can say that dark matter is extremely elusive.
Alternatively, perhaps gravity behaves differently at very low accelerations, as compared to predictions from general relativity and Newtonian gravity. This would be for accelerations less than c/T, where c is the speed of light and T is the age of the universe (currently a little under 14 billion years). This is much lower than our laboratory measurement capabilities.
Perhaps some form of the Modified Newtonian Dynamics concept (MOND), proposed by Milgrom in the 1980s, is correct. A specific recent proposal, by Dutch physicist Erik Verlinde, is known as emergent gravity. In his model, the interaction between dark energy and ordinary matter spoofs the existence of dark matter. To be clear, in this case there would be no dark matter, just extra gravity beyond what is expected, at the edges of galaxies and in galaxy clusters. That would lead to the greater velocities observed.
So what is dark energy? It is the residual energy of empty space, which is not completely empty. Imagine you take all the particles out of a region of space. There would still be a residual vacuum energy, expected from quantum mechanics principles, although we have trouble calculating what the strength of it should be.
This figure shows dark energy fraction (y-axis) vs. matter fraction (x-axis), for an expanding homogeneous universe and as measured with 3 different techniques using supernovae (blue), the cosmic microwave background (orange) and large scale galaxy distribution information (green). The small dark grey ellipse (within the blue ellipses) shows the overlap of the three techniques. There is good agreement for a model dominated by dark energy and with dark matter as the second largest contribution.
We observe its effects over very large scales, scales larger than galaxy clusters, at the level of superclusters and of the universe as a whole. We see its effects in the cosmic microwave background and from the patterns of galaxy spatial correlations at very large scales of half a billion light-years or more. Dark energy was first detected 20 years ago by looking at the distance-redshift relation for very distant supernovae of type Ia.
But the idea is as old as Einstein, who inserted a cosmological constant into his equations of general relativity. He called it his ‘greatest blunder’ but it turns out not to have been a mistake at all!
Dark energy can take different forms, but the important thing is that it has a negative pressure more than offsetting a positive energy. General relativity includes not only matter and energy terms, but also pressure terms. It turns out that the negative pressure of dark energy can end up ‘outweighing’ the positive energy and the result is a sort of negative gravity that drives an expansion or stretching of space itself.
And there is a particular form of dark energy that reproduces the same behavior as the cosmological constant of Einstein. It is constant in space and time and has a certain equation of state (relationship between pressure and energy densities) possessing a parameter w = -1/3. What we observe matches this.
Thus we have attractive gravity from ordinary matter, dark matter, and the energy content of dark energy. And we have repulsive gravity from the negative pressure content of dark energy. That final term outweighs all the others.
The basic accounting for the canonical cosmological model is 5% ordinary matter, 25% dark matter and about 70% dark energy, as the average densities across the universe. (one uses E = m*c^2 to normalize between matter and energy density).
The figure above shows the mass density contribution of around .3 on the x-axis and the dark energy density contribution of around .7 on the y-axis. Combining the constraints from supernova observations, from the large scale galaxy distribution (as a result of Baryon Acoustic Oscillations in the early universe) and the cosmic microwave background indicates a topologically flat universe that is 30% matter (mostly dark matter) and 70% dark energy, by mass-energy equivalence.
Thus we live in an accelerating, runaway universe dominated by dark energy. The dark energy began to dominate around 5 billion years ago as the matter density continually dropped. Dark energy is different, in that it retains the same density even as the universe expands, each unit volume has its ‘own’ dark energy.
The scale factor for the universe is growing exponentially.
It is on a course to double in size in each dimension every 12 billion years or so. One hundred billion years from now the volume of our presently visible universe will be (256)^3 as large as today, or about 16 million times larger!
Want to read more? See my blog at: https://darkmatterdarkenergy.com/blog/ or the book Dark Matter, Dark Energy, Dark Gravity available at amazon.com/books (search: Perrenod). 
Stephen Perrenod

Stephen Perrenod’s research area was clusters of galaxies and their X-ray emission. He holds Ph.D. and Master’s degrees in Astronomy from Harvard University and a Bachelor’s in Physics from MIT.

He created the first computer model for the evolution of X-ray clusters of galaxies over cosmological time scales.

After his career in astrophysics he moved into the high performance computing field, where he has worked for over a quarter century. He has been a frequent public speaker on HPC, Grid and Cloud computing topics. He wants to share his knowledge through this website for curious students and researchers

Stephen Perrenod
Stephen Perrenod’s research area was clusters of galaxies and their X-ray emission. He holds Ph.D. and Master’s degrees in Astronomy from Harvard University and a Bachelor’s in Physics from MIT. He created the first computer model for the evolution of X-ray clusters of galaxies over cosmological time scales. After his career in astrophysics he moved into the high performance computing field, where he has worked for over a quarter century. He has been a frequent public speaker on HPC, Grid and Cloud computing topics. He wants to share his knowledge through this website for curious students and researchers