Today’s New Reason To Believe

by Jeff Zweerink

What was discovered over 75 years ago, has not been detected using electromagnetic radiation, dominates the mass budget of the universe, and is filled with concrete?

Give up?

This mysterious substance is dark matter (I threw the concrete in just to make it hard).

However, a massive new simulation showing how dark matter behaves in galaxies may shed light on ways astronomers can detect gamma rays from dark matter collisions in our Milky Way Galaxy (MWG).

Although scientists do not understand the nature of dark matter, most believe that it is some sort of undiscovered subatomic particle. One popular candidate theory argues that each particle we know (like electrons, quarks, neutrinos, etc.) also has a supersymmetric partner particle. During the early history of the universe, these supersymmetric particles would have been produced in abundance. As the universe cooled, production would have ceased and the heavier particles have decayed to lighter supersymmetric particles. According to the theory, the lightest of these particles should be stable and, therefore, is one of the leading dark matter candidates.

However, even the lightest supersymmetric particle will annihilate when it collides with its antiparticle, and this annihilation will produce a pair of gamma rays. Thus, where the density of dark matter is large enough, astronomers should see a gamma-ray signal. Previous studies argued that clumps of dark matter in galaxies will produce the strongest signal. A recent simulation reveals that the smooth dark matter halo of a galaxy should give off the most gamma rays.

These results are important because NASA recently launched the Fermi Gamma-ray Space Telescope. One of the primary motivations for this mission was to “search for signs of new laws of physics and what composes the mysterious dark matter.” Knowing where to look and what sort of signal to expect greatly facilitates this search.

As it currently stands, dark matter represents one of the many parameters of this universe that exhibit fine-tuning. RTB expects future discoveries about the nature of dark matter (both from space and in particle accelerators) to reveal this design in more detail.

by Jeff Zweerink

History disfavors any theory placing Earth in a geometrically special location. Early scientists, such as Ptolemy of ancient Greece, thought Earth resided at the center of the solar system. However, geocentric cosmology eventually gave way to heliocentrism most notably associated with Nicolas Copernicus. Since Copernicus’s time extensive observations have demonstrated that the Sun does not reside at the center of the Milky Way Galaxy (MWG). Nor does the MWG reside at the center of the Local Group of galaxies or the universe. Scientists refer to the fact that Earth is not in a central, specially favored position as the Copernican Principle.

Although this view provides a foundation of cosmological research, scientists don’t simply accept the Copernican Principle. They continue to test it.

One area of research particularly suited to testing is the universe’s mysterious dark energy. The first need to invoke dark energy to explain features of the universe arose as astronomers tried to understand observations of distant Type Ia supernovae. The supernovae appeared dimmer than expected and the simplest explanation was that dark energy was causing the expansion of the universe to accelerate. However, dark energy is not the only explanation.

The same supernovae data would arise if the MWG resides at the center of a large region (something similar in size to the observable universe) with a lower density than that of the surrounding regions. However, placing the solar system (located within the MWG) at the center of such a special region clearly violates the Copernican Principle. Nevertheless, scientists do not simply reject the low density region, also called the void model. They seek to test its validity.

In a previous TNRTB I highlighted one test of the void model that used the cosmic microwave background. Now scientists have developed another test using supernovae data. Reseachers started by characterizing void density profiles that could explain the supernovae data. Then they modeled in detail how the supernovae data would appear with a much larger sample than currently exists. They found that with a sufficient sample of supernovae data from a specified distance, the void model produced different results when compared to dark energy models. Observations over the next few years should definitively tell which model is correct.

If dark energy models prevail, cosmologists will continue to face the great challenge of trying to understand what it is and why it exhibits such extraordinary fine-tuning in order for this universe to support life. If the void models prevail, a guiding scientific principle will need revision. Either way, exciting times lay ahead.

If you would like to see a question about the multiverse addressed in this forum, send it to multiverse@reasons.org.

by Jeff Zweerink

Next August, Reasons To Believe will host a conference on a week-long Alaska cruise onboard a Holland America ship. Those of you who plan to come along will enjoy all the comforts the ship offers as it sails along the beautiful Alaskan coast. With everything bearing the Holland America logo, imagine finding each room stocked with Royal Caribbean sheets and towels. Such evidence would indicate that other vessels beyond our own existed.

A team of cosmologists recently made a similarly shocking discovery when looking at our universe.

Since its detection in the 1960s, the cosmic microwave background (CMB) radiation has provided a wealth of information about our universe. This uniform, pervasive radiation points to a cosmic beginning to all of space and time. More recent studies of the CMB demonstrate that our observable universe exhibits a flat geometry and is dominated by a mysterious “dark energy.”

Perhaps the most remarkable discovery to come from studies of the CMB is that the observable universe appears to be moving in a particular direction!

The gravitational potential of galaxy clusters and their motion affected CMB photons as they passed through the clusters on their way to Earth; scientists refer to this as the Sunyaev-Zel’dovitch (SZ) effect. By measuring the SZ effect caused by the clusters’ motions—the kinematic SZ (or KSZ) effect—scientists can determine the direction and speed of the clusters. Although for individual clusters the KSZ is generally small and indistinguishable from noise, any coherent motion becomes detectable when analyzing a large number of galaxy clusters.

As reported in a recent article in the Astrophysical Journal, a team of scientists performed such an analysis. They’ve found that the entire sample of galaxy clusters is moving in the same direction with the same speed out to distances of six billion light-years. The most straightforward interpretation of this result is that gravitational effects occurring before inflation pulled the observable universe in one direction. Although inflation moved the mass/energy responsible for this “tug” far beyond our ability to detect it directly, the observable universe continues to move in the direction it was tugged.

If this result stands under further testing, it would add to the observational evidence that we live in a Level I multiverse. The authors of the article also note that this explanation may also account for the unexpectedly small values measured at the lower harmonics in the CMB radiation compared to predictions. As I described in a previous TNRTB, some have used this discrepancy to argue that the universe is basically the same size as the observable universe (which means no Level I multiverse).

To borrow an analogy from my colleague Kenneth Samples, many think multiverse furniture fits more comfortably in naturalists’ worldview. I have argued to the contrary. Not only does the multiverse not help naturalists, it finds a welcome home in a Christian setting.

If you would like to see a question about the multiverse addressed in this forum, send it to multiverse@reasons.org.