Posted on by Jean Tate

What is Loop Quantum Gravity?

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect. The animation below shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet

The two best theories we have, today, in physics – the Standard Model and General Relativity – are mutually incompatible; loop quantum gravity (LQG) is one of the best proposals for combining them in a consistent way.

General Relativity is a theory of spacetime, but it is not a quantum theory. Since the universe seems to be quantized in so many ways, one approach to extending GR is to quantize spacetime … somehow. In LQG, space is made up of a network of quantized loops of gravitational fields (see where the name comes from?), which are called spin networks (and which become spin foam when viewed over time). The quantization is at the Planck scale (as you would expect). LQG and string theory – perhaps the best known of theories which aim to both go deeper and encompass the Standard Model and General Relativity – differ in many ways; one of the most obvious is that LQG does not introduce extra dimensions. Another big difference: string theory aims to unify all forces, LQG does not (though it does include matter).

Starting with the Einstein field equations of GR, Abhay Ashtekar kicked of LQG in 1986, and in 1988 Carlo Rovelli and Lee Smolin built on Ashtekar’s work to introduce the loop representation of quantum general relativity. Since then lots of progress has been made, and so far no fatal flaws have been discovered. However, LQG suffers from a number of problems; perhaps the most frustrating is that we don’t know if LQG becomes GR as we move from the (quantized) Planck scale to the (continuum) scale at which our experiments and observations are done.

OK, so what about actual tests of LQG, you know, like in the lab or with telescopes?

Well, there are some, potential tests … such as whether the speed of light is indeed constant, and recently the Fermi telescope team reported the results of just such a test (result? No clear sign of LQG).

Interested in learning more? There is a lot of material freely available on the web, from easy reads like Quantum Foam and Loop Quantum Gravity and Lee Smolin’s Loop Quantum Gravity, to introductions for non-experts like Abhay Ashtekar’s Gravity and the Quantum, to reviews like Carlo Rovelli’s Loop Quantum Gravity, to this paper on an attempt to explain some observational results using loop quantum gravity (Loop Quantum Gravity and Ultra High Energy Cosmic Rays).

As you’d expect, Universe Today has several articles on, or which feature, loop quantum gravity; here is a selection What was Before the Big Bang? An Identical, Reversed Universe, Before the Big Bang?, and Before the Big Bang.

Source: Wikipedia

Posted on by Jean Tate

Stellar Parallax

Progress in astrometic accuracy (Credit: ESA)

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Parallax is the apparent difference in the position (line of sight to) an object, when the object is viewed from different locations. So, when we observe that a star has apparently moved (not to be confused with it actually having moved – proper motion), when we look at it from two different locations on the Earth’s orbit around the Sun (i.e. on different dates), that’s stellar parallax! (And if the star does not seem to have moved? Well, its parallax is zero).

The furthest apart two locations on the Earth’s orbit can be is 2 au (two astronomical units), as when observations of an object are taken six months apart. By simple trigonometry (geometry), the distance to the object being observed is just the length of the baseline divided by the tangent of the parallax angle (the angular difference in the two lines of sight) … and since parallax angles are extremely small for stars (less than one arcsecond), the tangent of the angle is the same as the angle. This gives a natural unit of distance for stars, the parsec … which is the distance at which an object has a parallax of one arcsecond when viewed from a baseline of one au.

There was a pretty hot competition, among astronomers, to be the first to measure the parallax of a star (other than the Sun), back in the 1830s; the race was won by Friedrich Bessell (remember Bessell functions?), in 1838, with a measurement of the parallax of 61 Cygni (0.314 arcsecs, in case you were wondering; two other astronomers measured the parallax of different stars in the same year).

To date, the most accurate parallaxes (~1 milli-arcsec) are the 100,000 or so obtained by the ESA’s Hipparcos mission (which operated between 1989 and 1993; results published in 1997) … Hipparcos stands for High Precision Parallax Collecting Satellite, but is also a nod to the ancient Greek astronomer Hipparchus. The follow-up mission, Gaia (target launch date: 2012) will substantially improve on this (up to a billion stars, parallaxes as small as 20 micro-arcsec). Here’s a fun fact: Gaia will measure the gravitational deflection caused the Sun … across the whole sky (and detect that due to Mars, for stars near the line sight to it)!

Universe Today has several stories on, or featuring, stellar parallax; here are a few: New Stellar Neighbors Found, Chasing an Occultation, and Happy Birthday Johannes Kepler.

Distance in Space is an Astronomy Cast episode on this very topic!

References:
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/parallax.html

Posted on by Jean Tate

What is Absolute Temperature?

If you measure temperature relative to absolute zero, the temperature is an absolute temperature; absolute zero is 0.

The most widely used absolute temperature scale is the Kelvin, symbolized with a capital K, which uses Celsius-scaled degrees (there’s another one, the Rankine, which is related to the Fahrenheit scale). We write temperatures in kelvins without the degree symbol; absolute zero is 0 K.

Another name for absolute temperature is thermodynamic temperature. Why? Because absolute temperate is directly related to thermodynamics; in fact it is the Zeroth Law of Thermodynamics that leads to a (formal) definition of (thermodynamic) temperature.

Roughly speaking, the temperature of an object (or similar, like the gas in a balloon) measures the kinetic energy of the particles (atoms, molecules, etc) of the matter it’s made up of … in an average sense, and macroscopically. Note that blobs of matter have far more energy than just the kinetic energy of the atoms in the blob – there’s the energy that holds the atoms together in molecules (if there are any), the binding energy of the nuclei (unless the blog is pure hydrogen, with no deuterium), and so on; none of these energies are counted in the blob’s temperature.

You might think that at absolute zero a substance would be in its lowest possible energy state, especially if it is a pure compound (or isotopically pure element). Well, it isn’t quite that simple … leaving aside zero point energy (something quite counter-intuitive, from quantum mechanics), there’s the fact that many solids have several different, stable crystal structures (even at 0 K), but only one with minimal energy. Then there’s helium, which is a liquid at 0 K (the solid phase of a substance has a lower energy than the corresponding liquid phase), unless under pressure.

The Kelvin is one of the International System of Units (SI) base units (there are seven of these), and is defined with reference to the triple point of water (“The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water” is the 1967/8 definition; the current one – adopted in 2005 – expands on this to take account of isotropic variations).

Why is it called the Kelvin? Because William Thompson – Lord Kelvin – was the first to describe an absolute temperature scale, in a paper he wrote in 1848; he also estimated absolute zero was -273o C.

Project Skymath has a nice introduction to absolute temperature.

Some Universe Today material you may find interesting: Absolute Zero, Coldest Temperature Ever Created, and Planck First Light.

Sources: Wikipedia, Hyperphysics

Posted on by Jean Tate

Pillars of Creation

One of the Hubble Space Telescope's most famous images, the "Pillars of Creation" in the Eagle Nebula. Credit: NASA/ESA

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The pillars of creation are a part of the emission nebula, or H II region, M16 (also called the Eagle Nebula).

The iconic Hubble Space Telescope image shown here was taken on April Fool’s Day, 1995, using the WFPC2 camera (you can tell it’s that camera from the W-shaped bite taken out of it). It was snapped as part of a research program by Arizona State University’s Jeff Hester and Paul Scowen, and released to the general public on 2 November (i.e. after the proprietary six-month period was over). Embryonic Stars Emerge from Interstellar “Eggs” – that’s the title of the HubbleSite Press Release; “eggs” is a play on EGGs, Evaporating Gas Globules, “dense, compact pockets of interstellar gas“. Interestingly, the name “pillars of creation” is found only in the image title, and nowhere in the Press Release text!

The pillars of creation – and M16 – are about 7,000 light-years away, and each are several light-years long (of course, there’s no “up” in space, so if you turn the image upside down, you see downward hanging linear features … but ‘stalactites of creation’ just isn’t at all catchy).

This region of M16 has been imaged in the x-ray region of the electromagnetic spectrum, by Chandra, in the infrared by Spitzer, and in infrared hi-def from the ground by the ESO’s VLT ANTU telescope.

Hubble has imaged many similar star-forming regions, complete with their own pillars; for example NGC 602 (in the Small Magellanic Cloud; zooming in on this image is fun – can you spot some of the ‘stalactites of creation’?), NGC 6357 (in our own Milky Way, just a tad further away than M16), and a different pillar (“Stellar Spire”) in the Eagle Nebula. Who knows? Maybe, one day, the Horsehead Nebula may become a pillar of creation too!

Universe Today has many articles on these pillars, Shadows Helped Form the “Pillars of Creation”, The Eagle … Has Arrived, Chandra Gives Another Look at the Pillars of Creation, Spitzer’s Version of the Pillars of Creation, and Eagle Nebula’s Pillars Were Wiped Out Thousands of Years Ago.

The Pillars of Creation also feature in Astronomy Cast episodes Nebulae, Stellar Populations, and Stellar Nurseries.

Posted on by Jean Tate

Megaparsec

velocity vs distance, from Hubble's 1929 paper

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A megaparsec is a million parsecs (mega- is a prefix meaning million; think of megabyte, or megapixel), and as there are about 3.3 light-years to a parsec, a megaparsec is rather a long way. The standard abbreviation is Mpc.

Why do astronomers need to have such a large unit? When discussing distances like the size of a galaxy cluster, or a supercluster, or a void, the megaparsec is handy … just as it’s handy to use the astronomical unit (au) for solar system distances (for single galaxies, 1,000 parsecs – a kiloparsec, kpc – is a more natural scale; for cosmological distances, a gigaparsec (Gpc) is sometimes used).

Reminder: a parsec (a parallax of one arc-second, or arcsec) is a natural distance unit (for astronomers at least) because the astronomical unit (the length of the semi-major axis of the Earth’s orbit around the Sun, sorta) and arcsec are everyday units (again, for astronomers at least). Fun fact: even though the first stellar parallax distance was published in 1838, it wasn’t until 1913 that the word ‘parsec’ appeared in print!

As a parsec is approximately 3.09 x 1016 meters, a megaparsec is about 3.09 x 1022 meters.

You’ll most likely come across megaparsec first, and most often, in regard to the Hubble constant, which is the value of the slope of the straight line in a graph of the Hubble relationship (or Hubble’s Law) – redshift vs distance. As redshift is in units of kilometers per second (km/s), and as distance is in units of megaparsecs (for the sorts of distances used in the Hubble relationship), the Hubble constant is nearly always stated in units of km/s/Mpc (e.g. 72 +/- 8 km/s/Mpc, or 72 +/- 8 km s-1 Mpc-1 – that’s its estimated value from the Hubble Key Project).

John Huchra’s page on the Hubble constant is great for seeing megaparsecs in action.

Given the ubiquity of megaparsecs in extragalactic astronomy, hardly any Universe Today article on this topic is without its mention! Some examples: Chandra Confirms the Hubble Constant, Radio Astronomy Will Get a Boost With the Square Kilometer Array, and Astronomers Find New Way to Measure Cosmic Distances.

Questions Show #7, an Astronomy Cast episode, has megaparsecs in action, as does this other Questions Show.

Posted on by Jean Tate

Exobiology

Exobiology (same thing as astrobiology) is about life in space (on other planets, and moons; in other solar systems): where it is, what it is, how it started, and how it evolved (all studied scientifically, of course). Because the origin of life right here on Earth, and its early evolution, is essentially unknown, and because of the distinct possibility of similiarities with the origin (and early evolution) of life elsewhere in the universe, exobiology includes research into abiogenesis (and early, and extreme, life on Earth).

Exobiology is very much a multi-disciplinary field, drawing on biology, chemistry, geology (and planetary science), physics, and astronomy.

Because we have a sample of just one – life on Earth – it is difficult to make anything but the most general decisions on what lines of exobiology research are likely to be productive (keep in mind that null results can, of course, be quite productive). Conservatively, looking for planets like Earth in orbit around stars like the Sun (in age as well as mass, metallicity, etc), and looking for clues for fossil life in planetary environments like those found today on Earth (e.g. early Mars) seem better options than investigating possible silicon-based life (to take just one example).

As the number of exosolar (or extrasolar) planetary systems known continues to grow, quickly, discovering the prevalence of Earth-mass planets, in goldilocks orbital zones, seems like a good idea … so today we have the Kepler mission and COROT.

As the early Mars becomes better understood – and the widespread distribution of liquid water then – so today we have plans for the Mars Science Laboratory and ExoMars (the discovery of methane in the Martian atmosphere certainly spurs such developments).

Less conservatively, the discovery of life around black smokers and sites like Lost City (not to mention entire ecosystems within crustal rocks … several km beneath the surface) sparked interest in the possibility of life in Europa, on Titan, even Enceladus (life – albeit rather simple life – we now know does not need to depend, ultimately, on the Sun’s (or another star’s) radiant energy … think chemolithoautotrophs).

Did you know that NASA has an exobiology branch? Check it out! Duke University’s Chemistry Department has an interesting Introduction to Exobiology you might find interesting too.

Universe Today stories on exobiology? Yep, lots; here’s a random selection: Martian Explorers Should Be Looking for Fossils, Did Life Arrive Before the Solar System Even Formed?, Extremophile Hunt Begins in Antarctica, Implications for Exobiologists , and New Targets to Search for Life on Europa.

Any Astronomy Cast episodes on exobiology? Yep … but it’s called Astrobiology.

Sources: NASA, ESA

Posted on by Jean Tate

Asterism

Kemble's Cascade (Credit: Walter MacDonald)

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The Big Dipper is an asterism (well-known to those who live in the northern hemisphere), so is the False Cross (well-known to those who live in the southern hemisphere). Asterisms are easily recognized pattern of *s*t*a*r*s* (but not a constellation).

The sky is full of asterisms easily seen without a telescope or binoculars: Summer Triangle, Great Square of Pegasus, the W in Cassiopeia, Frying Pan, Orion’s Belt, … it’s a long list.

The Southern Cross is not an asterism, strictly speaking, because it’s a constellation (Crux).

An asterism can take in parts of more than one constellation; for example, the Square of Pegasus has three stars in Pegasus (the three brightest, alpha, beta, and gamma Peg), and one in Andromeda (alpha And).

Some well-known asterisms are visible only through a telescope or binoculars; for example the Coathanger, and Kemble’s Cascade.

A couple (at least) of open clusters are also asterisms – the Hyades and the Pleiades (also known as the Seven Sisters).

Some clear, fixed features in the night sky, with well-known names, are not asterisms or constellations … the Coalsack for example, is a dark cloud in the plane of the Milky Way which blocks its light, and the Magellanic Clouds are dwarf, satellite galaxies of our own.

As astronomy in many cultures developed independently of the West (ancient Greece, Rome, etc), many of the commonly recognized constellations in those cultures correspond to asterisms … see if you can recognize some of the Chinese ones!

A particularly interesting kind of constellation is the dark constellation; instead of joining up bright stars to make an easily recognized figure, some cultures linked various dark nebulae in the Milky Way; for example the Emu in the Sky of the Australian Aborigines (and no, these are not asterisms).

SEDS (Students for the Exploration and Development of Space) has a concise list of asterisms easily visible without binoculars, or a telescope (though you may have to go to the opposite hemisphere to see them all!).

Asterisms are mentioned in many of Universe Today’s Weekend SkyWatcher’s Forecasts (August 21-23, 2009, for example), in its articles on Constellations (e.g. Orion), and Kids Astronomy ones (e.g. Finding the Summer Triangle).

Posted on by Jean Tate

Dwarf Star

A comparison of the Sun in its yellow dwarf phase and red giant phase

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A dwarf star is a star that is not a giant or supergiant … in other words, a dwarf star is a normal star! Of course, some dwarf stars are much smaller (less massive, have a smaller radius, etc) than normal (or main sequence, not really massive) stars … and these have names, like white dwarf, red dwarf, brown dwarf, and black dwarf. Our very own Sol (the Sun) is a dwarf star … a yellow dwarf.

Looking more closely at this rather confusing class of objects: a dwarf star has a mass of up to about 20 sols, and a luminosity (a.k.a. intrinsic brightness) of up to about 20,000 sols (‘sol’ is a neat unit; it can mean ‘the mass of the Sun’, or ‘the luminosity of the Sun’, or …!). So just about every star is a dwarf star! Why? Because most stars are on the main sequence (which means almost all have luminosities below 20,000 sols), and only a tiny handful of main sequence stars are more massive than 20 sols. In addition, once a star has burned through all its fuel, it becomes a white dwarf (and, one day, a black dwarf), all of which are dwarf stars by this definition.

The most interesting class of dwarf star is, perhaps, the black dwarf star; it’s hardly a star at all (it doesn’t burn any fuel, except, perhaps, deuterium, for a few million years or so).

So why do astronomers have this classification at all? Hitting the history books gives us a clue … back when spectroscopy was getting started, among astronomers – and well before there was any kind of astronomy except that in the optical (or visual) waveband; think the second half of the 19th century – a curious fact about stars was discovered: the spectra of stars with the same colors could still be very different (and when their distances were estimated, these spectral differences were found to track luminosity). So while dwarf stars overwhelmingly dominate, in terms of numbers, the giants (and sub-giants, and supergiants) pretty much rule in terms of what you can see with your unaided vision.

Neatly linking one kind of dwarf (the Sun, as a yellow dwarf) to another (white dwarf) is Universe Today’s The Sun as a White Dwarf. Other Universe Today articles on dwarf stars (not only white dwarfs!) include Astronomers Discover Youngest and Lowest Mass Dwarfs, Brown Dwarfs Form Like Stars, and Observing an Evaporating Extrasolar Planet.

Astronomy Cast’s episode Dwarf Stars has more on this topic.

Posted on by Jean Tate

Quintessence

Quintessence is one idea – hypothesis – of what dark energy is (remember that dark energy is the shorthand expression of the apparent acceleration of the expansion of the universe … or the form of mass-energy which causes this observed acceleration, in cosmological models built with Einstein’s theory of general relativity).

The word quintessence means fifth essence, and is kinda cute … remember Earth, Water, Fire, and Air, the ‘four essences’ of the Ancient Greeks? Well, in modern cosmology, there are also four essences: normal matter, radiation (photons), cold dark matter, and neutrinos (which are hot dark matter!).

Quintessence covers a range of hypotheses (or models); the main difference between quintessence as a (possible) explanation for dark energy and the cosmological constant Λ (which harks back to Einstein and the early years of the 20th century) is that quintessence varies with time (albeit slooowly), and can also vary with location (space). One version of quintessence is phantom energy, in which the energy density increases with time, and leads to a Big Rip end of the universe.

Quintessence, as a scalar field, is not the least bit unusual in physics (the Newtonian gravitational potential field is one example, of a real scalar field; the Higgs field of the Standard Model of particle physics is an example of a complex scalar field); however, it has some difficulties in common with the cosmological constant (in a nutshell, how can it be so small).

Can quintessence be observed; or, rather, can quintessence be distinguished from a cosmological constant? In astronomy, yes … by finding a way to observed (and measure) the acceleration of the universe at widely different times (quintessence and Λ predict different results). Another way might be to observe variations in the fundamental constants (e.g. the fine structure constant) or violations of Einstein’s equivalence principle.

One project seeking to measure the acceleration of the universe more accurately was ESSENCE (“Equation of State: SupErNovae trace Cosmic Expansion”).

In 1999, CERN Courier published a nice summary of cosmology as it was understood then, a year after the discovery of dark energy The quintessence of cosmology (it’s well worth a read, though a lot has happened in the past decade).

Universe Today articles? Yep! For example Will the Universe Expand Forever?, More Evidence for Dark Energy, and Hubble Helps Measure the Pace of Dark Energy.

Astronomy Cast episodes relevant to quintessence include What is the universe expanding into?, and A Universe of Dark Energy.

Source: NASA

Posted on by Jean Tate

Rigel

Rigel is the brightest star in the constellation of Orion; despite that, its formal name (one of them anyway) is Beta Orionis (Alpha Orionis – Betelgeuse – is a variable star, as is Rigel; Betelgeuse is sometimes the brighter, but most of the time is the fainter).

Rigel is a blue supergiant (spectral class B8I), the brightest of its kind in the sky. It’s also a multiple star system … the primary is the blue supergiant which totally dominates the observed light, and the secondary (Rigel B) is itself a close (spectroscopic) binary (B, and C, are both of B spectral class too … but are main sequence stars). HIPPARCOS data puts Rigel at a distance of ~850 light-years, but with a large uncertainty (GAIA will nail down its distance much more accurately).

Being a blue star, Rigel emits most of its light in the UV; if it is 850 light-years distant, its luminosity is approximately 85,000 sols, its radius ~75 sols (or ~0.35 au; if it were where the Sun is, Mercury would be almost inside it), its mass about 18 sols, and it is only approximately 10 million years old. It is likely to have a non-burning helium core (i.e. it is in its hydrogen shell-burning phase), and on its way to becoming a red supergiant (like Betelgeuse), and after that a supernova.

A couple of degrees away, on the sky, is the Witch-Head Nebula (IC 2118), which is a reflection nebula. And which star’s light is it reflecting? You guessed it, Rigel’s! Now as IC 2118 is about 40 light-years from Rigel, it demonstrates well just how much light Rigel is emitting.

Rigel may be part of the Orion OB1 association, if it were kicked out at around its birth (it’s too far, today, from the other stars in the association to be a member unless it is moving away at rather a fast clip).

Some of the Universe Today articles which feature Rigel include Rigel Passes Behind Saturn, Astrophoto: The Witch Head Nebula by Richard Payne, and IYA 2009 – Brian Sheen Reports on “Canoe Africa”.

Two Astronomy Cast episodes which relate to Rigel are The Life of Other Stars (in particular, the life of stars much more massive than the Sun), and Stellar Populations (in particular, the range of types of stars born from the same natal nebula).