Month: January 2016

The End of the World as We Know It

Centuries ago, Polynesian explorers settled on dozens of Pacific islands spanning from New Guinea to Hawaii to Easter Island.  Living on several islands provided the Polynesian culture a better chance for survival.  If disaster struck one island, the culture could still thrive on the other islands.  This is often, as recently expressed by Stephen Hawking, used as a rationalization for space colonization.  Is this a realistic model for human survival?  The best way to answer that is to understand how Earth protects life, what could endanger life on Earth, and how difficult it would be to migrate into space.

The Sun resides in a relatively quiet area of the galaxy referred to as the Local Bubble.  This bubble was created by a series of nearby supernovae events some 10-20 million years ago.  Even so, the Solar System is bathed with galactic cosmic rays and ionized solar winds which are harmful to life.  The Earth offers protective layers that insulates life from the harsh realities of space.  The magnetic field guides ionized particles towards the polar regions.  The harmful kinetic energies of these particles are absorbed by oxygen and nitrogen atoms in the upper atmosphere and converted to harmless light radiation in the form of the aurora.  The ozone layer blocks harmful ultraviolet (UV) rays from reaching the surface.  In fact, the upper atmosphere heats up at the ozone layer where the high energy UV radiation is absorbed.

Stratospheric heating (red) caused by ozone layer absorbing UV rays. Credit: ESA

The Earth’s atmosphere also absorbs high energy x-rays and gamma rays.  This is a key point as our first attempt to colonize space will most likely be on Mars.  And Mars does not offer the protective layers that Earth does from the harmful radiation of space.  Any attempt to colonize the red planet will need to invent technologies to provide protection from space radiation.  Also, Mars has only 1/3 the gravity of Earth which can deteriorate body muscle throughout a long duration stay.  We cannot change Mars’ gravity and for this reason, some propose to make future Mars missions a one way colonization effort* as returning to Earth’s gravity may be problematic.

How feasible is it to colonize Mars?  To put it in perspective, it is much easier to colonize Antarctica.  Currently, there are a few dozen scientists who occupy the South Pole station in any given year.  Going to Mars is possible, but during the next few decades only a handful, at best, will occupy our nearest neighbor.  When evaluating possible disaster scenarios on Earth, what type of timeline are we looking at?

For now, I want to focus on natural, rather than human induced, disasters.  First on deck are supervolcanoes.  A supervolcano is an eruption that releases at least 500 cubic km of magma.  By comparison, this is 500 times larger than the Mt. St. Helen’s eruption in 1980.  These events are pretty rare, about once every 100,000 years.  To put that in perspective, the Pyramids of Ancient Egypt were built 5,000 years ago.  The last supervolcano eruption was Lake Toba 74,000 years ago in Indonesia.  These events can lower global temperatures 10 degrees Celsius over a period of ten years.  Once believed to have reduced the human population to 11,000, new evidence suggests that Lake Toba was not as catastrophic to humanity as originally thought.

While that is good news, a supervolcano would certainly be disruptive to human civilization.  And while the chances are such an eruption in the near future are remote, at some point in time there will be one.  One such mantle hotspot resides in Yellowstone.  Recently, the Yellowstone magma chamber was mapped.

Credit: Hsin-Hua Huang, University of Utah.

The newly found lower chamber contains about 11,000 cubic km of magma.  The last Yellowstone eruption occurred 640,000 years.  The chances of an eruption in the near future is very slim.  As destructive as these events can be, it appears that the next such event could be 10,000 or more years in the future.  At this point, there seems little that could be done to thwart the threat of a supervolcano.  That is not the case with the danger of an asteroid/comet strike.

Impact events are not uncommon.  Small meteors collide with Earth everyday, usually burning up in the atmosphere.  When they are large enough to survive the frictional forces of the atmosphere, they strike the ground and are then called meteorites.  These objects are collector items but also valuable for scientific research.  Unlike the Moon, erosion typically wipes away evidence of past large impact events.  One exception is in Arizona where the dry climate has kept intact a 1,300 meter wide crater for 50,000 years from a 30-meter meteor impact event.

Arizona Meteor Crater and Visitor Center.  Credit: Shane Torgerson/Wiki Commons

If a impact is large enough, sizable amounts of material can be ejected into the atmosphere causing global cooling and potential danger to life.  Most famously, an impact near the Yucatan Peninsula 65.5 million years ago killed off the dinosaurs.  This was one of the largest impacts in the inner Solar System since the heavy bombardment formation stage some four billion years ago.  The cause of this was an object 10 km wide.  How often does such an event take place?

Fortunately, extinction type impacts are very rare.  In fact, the impact causing the dinosaur extinction is the last known event of this magnitude.  More common are smaller, but still damaging impacts such as the 1908 Tunguska event in Siberia.  In this case, a 120 foot object vaporized some 5 miles above the ground and the concussion was felt dozens of miles away.    While potentially devastating on a local scale, these impacts would not present a threat to humanity on a global basis.  Impacts of this scale occur around once every 300 years.

Trees knocked over by Tunguska impact. Credit: Leonid Kulik

Unlike supervolcanoes, it is feasible to mount a defense against a possible asteroid or comet impact.  NASA now has Planetary Defense Coordination Office whose mission is to locate, track, and devise efforts to defend against collisions with Near Earth Objects (NEO).  NASA has discovered over 13,000 NEO’s and detects an additional 1,500 NEO’s per year.  The program’s budget is $50 million annually.  That is 25% the cost to make the most recent Star Wars movie.  NASA’s goal is to have a test mission to redirect an asteroid during the 2020’s.  While we can plan to defend against impact events, the stellar evolution of the Sun is much more problematic.

The Sun is an average sized main sequence star halfway through its expected lifespan of 10 billion years.  Main sequence stars like the Sun fuse hydrogen into helium in their cores.  Over the next billion years, the Sun will become hotter and more luminous.  As a star ages, the rate of fusion in its core rises.  Some 3.5 billion years from now, the Sun will emit enough energy to vaporize the oceans and propel Earth’s remaining water vapor into space.  And it doesn’t stop there.  About 5 billion years from now, the Sun’s core will run out of hydrogen and begin fusing helium into carbon.  The core will become hotter causing the Sun to expand into a red giant.  At this point, the Sun will consume the inner planets including Earth.  Here, humankind will need to develop interstellar travel or cease to exist.

Would interstellar travel guarantee our survival as a species?  Not quite.  The universe itself is evolving and has a life span.  Currently, the universe is expanding at an accelerating rate.  If this trend were to continue, 22 billion years from now some models predict the Big Rip will occur.  In this state, all matter down to sub-atomic particles will have been shredded apart, making life in our universe impossible.  Unlike stellar evolution, the eventual outcome of the universe is not completely known.  While we can observe other sun-like stars to see how they live and die, we do not have the ability to observe other universes to do the same.  And in fact, we do not even know what 95% of our own universe is made of.  Nonetheless, physicists, such as Michio Kaku, have floated proposals for life to escape to a parallel universe when ours becomes uninhabitable.

So, how should we plan for the future of humanity and where do we place our priorities?  Lets take a look at a potential timeline of possible threats.

Climate change:               0-100 years

Nuclear proliferation:    0-100 years

Supervolcanoes:              0-30,000 years

Impact:                              0-tens of millions of years

Sun:                                    3.5 billion years

Universe:                          Tens of billions of years

The most imminent threats are human made, rather than natural.  It is technically feasible to defend against meteor/comet strikes while not the case with supervolcanoes.  More than likely, that leaves us with a few thousand years to figure out how to establish a permanent human presence on Mars.  Certainly, going to Mars is doable if the incentive is there to devote resources and funding.  It will not be possible to defend Earth against the Sun’s stellar evolution.  If interstellar travel is possible, and that’s a big if, we have on the order of a billion years to find a way to do that.  Like a lot of space enthusiasts, I’d like to see that happen in the 23rd century just as in Star Trek.  However, unlike exploring the Solar System, the distances involved with interstellar travel will require a far-reaching advancement of physics and engineering that is not guaranteed to happen.  So what do we do now?

Hollywood blockbusters notwithstanding, the major priority should be getting a handle on human induced dangers such as climate change and nuclear proliferation.  Concurrently, we can continue our efforts to begin human exploration of Mars.  All this can take place in the next century but it must be stressed a human settlement on Mars is not a substitute for cleaning up our act on our home planet.  During this time, we will begin to discover Earth-like planets, and possibly, life beyond our Solar System.

Efforts to begin interstellar exploration are in a very, very prototype stage.  Relativity places a limit on velocity at the speed of light.  Concepts to bypass this limit are speculative at best and are why, as mentioned earlier, will require a deeper understanding of physics to accomplish.  This advancement will have to be on the order of what Issac Newton achieved in the late 1600’s and modern (both Einstein and the quantum physicists) physics in the 20th century.  When and if this happens, we can plan for humanity’s migration to the stars to escape the Sun’s vaporization of Earth.  If we are fortunate enough to accomplish that, we can owe it to the same spirit that carried the Polynesians in their wooden canoes across the vast expanse of the Pacific.

*To be clear on this point, the Mars One initiative is not realistic in its timeline or funding.  However, other proposals, such as offered by Buzz Aldrin, may be more realistic.  

**Image on top of post is the famous Earthrise photograph from Apollo 8, the first space mission to carry humans to another celestial object.  Taken on Christmas Eve, 1968.  Credit:  NASA.

The Dark Universe

During the 1930’s, Mt. Wilson Observatory was famous for the revolutionary work of Edwin Hubble.  Galaxies were discovered to exist outside the Milky Way and the universe was found to be expanding.  In 1931, Einstein would visit the observatory and at Caltech, listen to a seminar by Fr. Georges Lemaitre on the theory of the origins of the universe, later dubbed the Big Bang.  The founder of observatory, George Ellery Hale, was busy working on the successor to the 100-inch Mt. Wilson telescope.  In 1934, the 200-inch mirror was cast, with a great amount of public fanfare for the future observatory at Mt. Palomar.  Meanwhile, at the same facility, under the radar of the media, Fritz Zwicky was unearthing one of the great mysteries of astronomy today.

Fritz Zwicky. Credit: California Institute of Technology.

The Coma Cluster consists of some 1,000 galaxies at a distance of 320 million light years from Earth.  The cluster itself is about 20 million light years wide.  In 1933, Zwicky published a study of the cluster which indicated its mass was much greater than its visible content could account for.  Had the optical mass of the cluster was all that existed, the velocities of the galaxies would have exceeded the escape velocity of the cluster, meaning there would not have been a cluster at all.  Zwicky realized there must have been mass in the cluster that could not be seen.  It was this extra mass that increased the escape velocity of the cluster keeping it intact.  The results were originally published in a Swiss journal Helvetica Physica Atca (Zwicky was originally from Switzerland).  Zwicky dubbed the invisible mass dunkle materie, or dark matter.

The Coma Cluster. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).
Acknowledgment: D. Carter (Liverpool John Moores University) and the Coma HST ACS Treasury Team.

It would take a few decades for the concept of dark matter to gain traction in astronomy.  Part of it was new technology needed to be developed before it could be researched further.  Part of it was Zwicky was ahead of his time.  Besides dark matter, Zwicky developed the groundwork for ideas such as supernovae, neutron stars, and galactic gravitational lenses.  Familiar to astronomers today, these were radically advanced concepts during the 1930’s.  Zwicky also had, even by the standards of academia, a contentious personality towards other astronomers.  Most famously, Zwicky referred to his fellow astronomers as “spherical bastards”.  Why?  They were, according to Zwicky, bastards any way you looked at them.  Needless to say, this did not endear Zwicky to other astronomers, who were reluctant to promote Zwicky’s ideas.

Those issues aside, one by one, Zwicky’s theories received observational confirmation.  Zwicky would discover 120 supernovae, the most by any astronomer.  The first neutron star was discovered in 1967 with the radio detection of a pulsar in the Crab Nebula.  The first image of gravitational lensing of a quasar occurred in 1979.  As for dark matter, it would take the efforts of Vera Rubin, who faced bias against her work, to provide verification of its existence.

Rubin
Vera Rubin in 1967, when she became the first women allowed to observe at Mt. Palomar. To the right is Konrad Rudnicki. Credit: Carnegie Institution for Science.

Unlike Zwicky, Vera Rubin did not give cause for astronomers to disdain her work.  Astronomy has been traditionally a male dominated field.  Many top graduate astronomy departments did not admit women until the 1970’s, and it was Vera Rubin who became the first women to observe at Mt. Palomar in 1967, nearly two decades after it had seen first light.  The overall bias against women caused astronomers to greet Rubin’s early work with skepticism, and in some cases, downright hostility.  In the end, the high quality of Rubin’s work would break through these barriers.

During the 1970’s, Rubin and her colleague Kent Ford, embarked on a study of galactic rotation curves.  Kent Ford was responsible for building spectrographs sensitive enough to detect the Doppler shifts of stars as they orbited around a galactic core.  Rubin decided upon this program for among other things, it would not require as much telescope time to complete as she had to balance her career with her family life.  The expected result was that the farther out a star was from the center of mass in a galaxy, the slower its velocity would be.  When it came time to bump the data against the model, the results came out like this.

Credit: Queens University.

This mirrored Zwicky’s study of the Coma cluster four decades earlier.  If the optical mass was all there was to the galaxy, the stars at the outer edge were going so fast they would escape the gravity of the galaxy.  However, that was not the case and there was much more dark matter holding these galaxies together than luminous matter.  In fact, Rubin’s measurements indicated that only 10% of galactic mass was of the visible variety, and 90% was dark matter.  Thus, 90% of galaxies were made of stuff that astronomers had no idea what it was.  This was a staggering revelation.

When a discovery such as this that runs so counter-intuitive to the expected result, it will typically come under very critical review.  That’s a good thing and a necessary part of the scientific process as long as it is the data being scrutinized and it does not become a personal matter.  The best way to rebut criticism of a discovery is to provide replication.  In 1970, Rubin and Ford published the flat galactic rotation curve for the Andromeda Galaxy.  Throughout the decade, astronomers sought out a solution for the rotational curve without dark matter.  However, by 1978, dozens of rotational curves reproduced the original result and hundreds more would follow the next decade.  The dam had broke and by the time I was an undergrad student in the early 1980’s, dark matter had become standard material in galactic astronomy courses.

I distinctly remember the shock of learning 90% of the universe was made of dark matter whose nature was not known, but whose gravitational effects were clearly observed.  In 1998, astronomy was rocked again by the discovery that matter, both luminous and dark, itself comprised only 20-25% of the universe.  And this discovery would again trace its roots to Fritz Zwicky.

In 1937, Zwicky discovered a supernova that was distinctly different from what he had observed before.  This supernova was brighter, and faded at a slower rate.  Unlike the other supernovae, this was not the death throes of a high mass star.  This was an explosion of a white dwarf that was siphoning mass from a companion star.  Six years earlier, Subrahmanyan Chandrasekhar determined the maximum mass a white dwarf can obtain is 1.4 Suns.  Once a white dwarf tops this amount of mass, its dense (a teaspoon of a white dwarf weighs 15 tons) carbon rich core ignites and creates a supernova.  Since white dwarfs are the same mass when they explode, the brightness of these events are roughly identical.  This gives astronomers a “standard candle” to calibrate distance.  These events were eventually referred to as a Type 1a supernova.

Subrahmanyan Chandrasekhar won the Nobel in 1983 for his work on stellar interiors. The Chandra X-Ray Observatory was named in his honor. Credit: University of Chicago.

During the late 1990’s, two teams of astronomers were competing to measure how gravity slowed the expansion of the universe since the Big Bang.  The expectation was over time, gravity would rein in the rate of expansion.  The way to determine this is to measure the red shift of Type 1a supernovae.  As an object races away from Earth, its spectrum is shifted towards the red.  The faster it is moving away, the greater the red shift.  As these supernovae serve as a standard candle, their distance could be determined.  The farther away these events were, the older they occurred as it would take longer for their light to reach Earth.  Thus, the goal was to utilize Type 1a supernovae to measure the expansion of the universe throughout its history.

In 1998, the High-Z Supernova Search Team and the Supernova Cosmology Project independently released their results.  What they found was not only did gravity fail to slow the expansion of the universe, but the expansion was accelerating.  Gravity, obviously, was still around, but there was a mysterious force in the universe that not only counteracted gravity, but was of increasing presence as the universe expands.  This force was referred to as dark energy and makes up some 70% of the universe.

What was discovered as well is the universe, like stars and galaxies, has evolved over time.  Up to 5 billion years ago, gravity did slow down the expansion of the universe.  After that epoch, dark energy became a stronger force and the expansion began to speed up.  To put that time into perspective, the Earth and the Solar System is 4.5 billion years old.  The timeline of the expansion is shown below.

Credit: NASA/ESA and A. Feild (STScI).

As the expansion accelerates, some hypothesize that the universe will end in what is called the Big Rip.  This refers to a state where the universe expands to the point where stars, planets, and even atomic particles are shredded apart.  To be sure, life could not exist in such a universe.  However, there is no need to worry, the latest estimate is a Big Rip would take place 22 billion years from now.  And at any rate, we’ll need to learn more about what exactly dark energy is before we arrive at a definitive theory on the ultimate fate of the universe.

Currently, the South Pole Telescope is dedicated to researching dark energy.  Built by the University of Chicago, the 10-meter telescope is dedicated to locating galaxy clusters for the purpose of mapping cluster formation throughout the life of the universe.  It is hoped this effort will provide further answers on how both gravity and dark energy has shaped the expansion of the universe.  The future of dark energy research remains in the Southern Hemisphere but in Chile.

The Vera Rubin Observatory is currently under construction in Northern Chile and is expected to be operational by 2024.  The cost of the Rubin Observatory is estimated at $465 million, about half the cost of a new NFL stadium.  Funded by a combination of public and private sources, this 8.4 meter wide angle telescope is designed to have the ability to survey the entire sky in three nights.  The wide field of view will enable the Rubin Observatory to map large scale galactic structures and survey Type 1a supernovae over a ten year period. This will provide a more comprehensive map of how dark energy and dark matter have influenced the overall structure of the universe throughout time.

Rubin Observatory under construction with Milky Way overhead. Credit: Bruno C. Quint/Rubin Observatory CC 4.0.

Given the dramatic nature of the discovery of dark energy, dark matter has been a bit overlooked since 1998.  However, there have been some key advancements in the study of dark matter during that time.  The Hubble Space Telescope was able to map dark matter by measuring gravitational lensing.  Gravity bends light rays and as a result, distorts galactic images.  While dark matter cannot be seen its gravitational effects can be observed.  What was found is that dark matter had a smoother distribution during the early universe but is more clumpy now.  This clumpiness creates a scaffolding for which galactic clusters form upon.

In 2015, dark matter was detected interacting with itself.  In a galactic collision in the cluster Abell 3827, dark matter was detected to lag behind normal matter.  This could provide a key clue as to what dark matter is as previously it was only detected to interact gravitationally.  If “dark forces” among dark matter exist causing this interaction, then this discovery may help physicists model dark matter in a more complex and accurate manner.  Ideally, this will help determine what exactly dark matter is.

So where does our knowledge of the universe stand now?  The recent ESA Planck mission mapped out the cosmic microwave background (CMB).  The CMB is the remnant of the Big Bang and shows us the state of the universe when it was 380,000 years old.  By measuring the fluctuations in the CMB, astronomers were able to breakdown the universe as follows:

Dark Energy:  68.3%

Dark Matter:  26.8%

Ordinary Matter:  4.9%

So the matter we see around us, the Earth, Sun, stars and galaxies, only comprise slightly less than five percent of the universe.  The remaining 95% is made of stuff that is unknown to us.  That, along with the question of life beyond Earth, represents the most important mystery for astronomers to solve in the upcoming decade.

*Image on top of post is the galactic cluster Abell 2218.  Where there is a galactic cluster, there is dark matter.  The gravity of both ordinary and dark matter deflects light just like a lens does.  The result is galaxies behind the cluster are magnified and distorted.  The magnification effect allows astronomers to image galaxies far away behind the cluster that ordinarily would not visible.  Credits: NASA, Andrew Fruchter and the ERO Team [Sylvia Baggett (STScI), Richard Hook (ST-ECF), Zoltan Levay (STScI)] (STScI).