A Giant Hole in the Universe?

Eridanus is a large constellation which has been known since ancient times.   The constellation begins in the north (near Orion’s left foot) then winds south across the sky before snaking west towards Cetus the sea monster.  The river of stars which makes up Erdanus then doubles back east and eventually ends far to the south at the border with Hydrus, the water snake.  Because of its antiquity, there is some dispute as to where the name Eridanus came from:  second-century Greek astronomers believed the name indicated the sacred (mythical) river which Phaeton plummeted into after his unhappy attempt to drive the chariot of the sun.   Other etymologists, however, think that the name originated in ancient Mesopotamia where “the star of Eridu” was sacred to the primeval god Enki, lord of the abzu, a mythical abyss filled with all the fresh water in the world.  Eridu was the first known city of Earth, so the name may go back to the origin of civilization.

Enki in his watery home, the Abzu

Whatever the origins of the name are, the constellation is the site of one of the strangest and most controversial objects in the heavens.  In 2007, astronomers using radio telescopes to survey the universe were astonished to discover nothing.  More specifically they discovered an immense and disconcerting amount of nothing—an enormous void in space time more than a billion light-years in diameter.  The Eridanus supervoid lies between six to 10 billion light-years away and its existence seems to be at odds with current cosmological models.

The Eridanus Supervoid (from an article by Bert Stevens)

Cosmologists have several schools of thought concerning how the supervoid came into being and what its real nature is.  Because I am having trouble understanding any of these crazy theories, I have provided a rudimentary metaphor for each in blue (which would probably offend cosmologists, if they were reading my blog).

1)      Supporters of the standard model Big Bang theory say the region is colder because of dark energy, a hypothetical form of energy believed to permeate all of space.  If it exists, dark energy uniformly fills otherwise empty space yet interacts with none of the known forces in the universe (save gravity). The void is not empty but is filled with dark energy–which we do not yet understand: just like an empty room would seem empty to the Babylonians (despite being filled with air to us).

2)      A contrary theory proposes that the known universe orbits a supermassive black hole (in the same fashion that galaxies spiral around central black holes). This explanation would explain the “accelerating/expanding” universe as a sort of illusion: objects on the edge of the universe would be orbiting at a greater velocity than objects close to the black hole—a phenomenon which would affect their red shift relative to us.  Of course anything that got too close to the black hole in the void would be swallowed to an unknown doom into a black hole with the mass of another universe.  The universe is like an old vinyl record being spun around by a black hole in the center which is enormous beyond comprehension.  The expansion of the universe is an illusion caused by our limited perspective in such a scenario.   

3)      Laura Mersini-Houghton, a cosmologist who theorizes about the multiverse, believes that the supervoid is the imprint of another universe beyond our own.  Quantum entanglement has allowed us to see a shadow of this parallel universe in the form of the great empty spot located in Eridanus.  ??? Um, there are other universes out there which interact with our own in unknown ways which cause big holes (or maybe windows).

4)      Conservative astronomers speculate that the empty spot is an anomaly of the cosmic texture of the early universe.  Phase transition after the big bang resulted in heterogeneous distribution of matter. The universe is like a loaf of bread—sometimes it just has big holes in it because of the way it came into being.  

5)      The radiometric finding method by which the void was discovered is flawed.  The area only seems anomalously “cold” (in terms of EM emissions) because of a relatively hot ring of emissions surrounding it. The void doesn’t exist.  It was a mistake in observation.

6)      Something else entirely which we don’t yet comprehend and haven’t even imagined. Something else entirely which we don’t yet comprehend and haven’t even imagined.

I’ll be honest here.  Since I don’t have a radio telescope array or a degree in theoretical physics, these ideas are pretty hard to assay.  They are also wildly divergent.  I am therefore going to evaluate them aesthetically/emotionally (i.e. uselessly) in the following manner.  The first idea has the support of the astrophysics community, but is unsatisfactory until we have a more-than-theoretical understanding of dark energy (which could be forthcoming because of our discovery of the Higgs Bosun).  The second idea seems like it could be tested with mathematical modeling and astronomical observation (which so far seem to indicate there is no giant black hole in the middle of everything).  The third idea seems insane—and yet I have always intuitively felt that there are universes beyond this one (I’m sorry to be so guilty of such magical/hopeful thinking).  The fourth and fifth ideas seem quite plausible because they are boring (although why is the universe leavened like bread? Or why does it contain large relatively hot rings?).  The sixth idea is always applicable to everything.

Horses and Birds (M. C. Escher, 1949, wood engraving)

Of course all this speculation may all be moot:  a more recent survey of the southern sky from a radio telescope in Australia suggests that there might be a much larger 3.5 billion light-year-wide void in the known universe.  That would certainly steer us back toward more conservative models of the universe, while at the same time leaving us with yet more questions.

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Sagittarius A

The constellation Sagittarius (from “Urania’s Mirror” a set of constellation cards published in England circa 1825)

My apologies for the blogging break last week.  Usually I try to write a new post every weekday, but last week was a blogging holiday.  To reinvigorate things after the lost week, let’s turn to a big subject—in fact a super-massive subject!  Long ago, Ferrebeekeeper featured a post about Eta Carinae, a blue hypergiant with a hundred times the mass of the sun (which is itself a million times more massive than Earth).  Stars like Eta Carinae are rarely formed and short lived—there are probably less than a dozen in our galaxy.  However compared to the most massive object in the galaxy, Eta Carinae is puny and common.  Twenty six thousand light years away from the solar system there exists a truly monstrous space object!

In 1974, Astronomers discovered an astronomical feature which was emitting exotic radio waves in the Sagittarius constellation. The scientists named the feature “Sagittarius A” and set out to determine what it was.  Part of the feature seems to be the remnants of a star which had gone supernova.  A second part of the feature is a cloud of ionized gas surrounded by an even larger torus of molecular gas.  In the middle of Sagittarius A is something which is emitting most of the high energy electromagnetic radiation visible to radio telescopes.  The cloud of ionized gas seems to be emptying into it and nearby stars orbit it with greater velocity than stars move anywhere else in the galaxy (in fact the object affects the proper motion of thousands of nearby stars).  And yet the space object at the center of Sagittarius A has a diameter of only 44 million kilometers–a bit less than the distance between the middle of the sun and Mercury at its perihelion (when the rocky planet is closest to the sun).  By calculating the proper motion of thousands of nearby stars, scientists determined that the mysterious object at the center of Sagittarius A (which they took to calling Sagitarrius A*) has mass of 4.31 million suns (i.e. solar masses). Whatever lies at the center of Sagittarius A–which I probably should have mentioned, is also the center of the Milky Way Galaxy–is smaller in volume than a large star, but has a mass which exceeds by many orders of magnitude even exotic hypergiants like Eta Carina.

Of course the only kinds of discrete objects which we know (or even hypothesize) to be capable of attaining such mass are black holes.  It is believed that most (indeed probably all) galaxies have super-massive black hole at their centers.  Smaller galaxies have small super massive black holes (forgive the oxymoron) but large galaxies have immense central black holes which can equal billions of solar masses.  Radio astronomers have observed plumes of exotic electromagnetic radiation coming from the center of other galaxies, and they wondered where the Milky Way’s galactic center was located.  It seems that a supernova near the galactic center blew away a great deal of the dust and gas on which the black hole would otherwise “feed” thereby making the galactic center of the Milky Way less energetic than the active center of farther (e.g. older) galaxies.

Artist’s Conception of Galactic Center

The super massive black holes which lie at the center of galaxies may be a result of the accretion of matter around stellar-sized black holes (which could grow quickly in matter-rich galactic cores) but most astrophysicists believe they are instead a primordial feature of the Big Bang around which galaxies themselves coalesced.  The ultimate nature of super massive black holes remains unknown and seems to be tied to the nature and shape of our universe.