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Hidden in plain sight — dark matter

Space.com posted an article on June 15, 2017, which is a good summary of this topic — “What is dark matter?

Roughly 80 percent of the mass of the universe is made up of material that scientists cannot directly observe. Known as dark matter, this bizarre ingredient does not emit light or energy. So why do scientists think it dominates?

Most scientists think that dark matter is composed of non-baryonic matter.

If scientists can’t see dark matter, how do they know it exists?

The article summarizes some experiments trying to detect this stuff.

Another useful source for background on this topic is Dan Hooper’s book.

Hooper, Dan (2009-01-09). Dark Cosmos:In Search of Our Universe’s Missing Mass and Energy. HarperCollins. Kindle Edition. [Copyright 2006, prior to the LHC going online and Planck satellite]

Wiki:

Dark matter is a hypothetical type of matter distinct from baryonic matter (ordinary matter such as protons and neutrons), neutrinos and dark energy. The existence of dark matter would explain a number of otherwise puzzling astronomical observations. The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum. Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter,gravitational lensing, its influence on the universe’s large-scale structure, on galaxies, and its effects in the cosmic microwave background.

The apparent expansion of the universe boggles the mind:

… studies of distant supernovae revealed that the universe today is expanding faster than it was in the past, not slower, indicating that the expansion is accelerating.

 

3 thoughts on “Hidden in plain sight — dark matter

  1. Space.com followed their June 15 article with another one on July 17, 2017, featuring an “Expert Voice” summary of the topic by Don Lincoln (Senior Scientist, Fermi National Accelerator Laboratory; Adjunct Professor of Physics, University of Notre Dame): Is Dark Matter Real?

    However, the reality is that dark matter’s existence has not yet been proved. Dark matter is still a hypothesis, albeit a rather well-supported one. Any scientific theory has to make predictions, and if it’s right, then the measurements you do should line up with the predictions. The same goes for dark matter. For instance, dark matter theories make predictions for how fast galaxies are rotating. But, until now, measurements made of the detailed dark matter distribution at the center of low mass galaxies didn’t line up with those predictions.

    A recent calculation has changed that. The calculation helps resolve the conundrum of the Tully-Fisher relation, which compares the visible, or ordinary, matter of a galaxy to its rotational velocity. In very simplified terms, scientists have found that the more massive (and therefore brighter) a spiral galaxy is, the faster it spins.

  2. Space.com published another article on Dark Matter today (August 14, 2017), again featuring an “Expert Voice” summary by Don Lincoln (Senior Scientist, Fermi National Accelerator Laboratory; Adjunct Professor of Physics, University of Notre Dame): Is Dark Matter Less ‘Lumpy’ Than Predicted?

    The Dark Energy Survey (DES) is a collaboration of about 400 scientists who have embarked on a five-year mission to study distant galaxies to answer questions about the history of the universe. It uses the Dark Energy Camera (DEC) attached to the Victor M. Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in the Chilean Andes. DEC was assembled in the U.S. at Fermilab near Batavia, Illinois, and is a 570-megapixel camera able to image galaxies so far away that their light is a millionth as bright as the dimmest visible stars.

    Gravitational lensing … Because the mass of the lensing galaxy is a combination of visible matter and dark matter, gravitational lensing allows scientists to directly observe the existence and distribution of dark matter on scales as large as the universe itself. This technique also works when a large cluster of foreground galaxies distorts the images of clusters of even more distant galaxies, which is the technique employed for this measurement.

    The DES collaboration recently released an analysis using exactly this technique. The team looked at a sample of 26 million galaxies at four different distances from Earth. … As expected, they found that the dark matter of the universe was “lumpy.” However, there was a surprise — it was a little less lumpy than previous measurements had predicted.

    Is that a big deal? Maybe. The uncertainty, or error, in the two measurements is big enough that it means they don’t disagree in a statistically significant way. What that simply means is that no one can be sure that the two measurements really disagree. It could be that the discrepancies arise by chance from statistical fluctuations in the data or small instrumental effects that were not considered.

    Even the study’s authors would suggest caution here. The DES measurements have not yet been peer-reviewed. The papers were submitted for publication and the results were presented at conferences, but firm conclusions should wait until the referee reports come in.

  3. Space.com published another article on Dark Matter today (August 25, 2017), this time featuring Paul Sutter, host of Ask a Spaceman, We Don’t Planet and COSI Science Now. The article is titled “The Matter with Dark Matter” and discusses the major evidence for its existence: too hot, too fast, too bumpy, too wide.

    As much as we would prefer to live in a simpler universe, dark matter is not the product of some astronomer’s fever dream after a late-night observing session. It’s only after decades of careful observations that cosmologists have come to the inescapable conclusion that most of the matter in our universe is simply invisible.

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