What Does The Expansion Of The Universe Tell Us About The Future?

We know the speed of light is constant, but what about the speed at which the universe is expanding?
Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe’s expansion to accelerate. Image credit: NASA/JPL-Caltech

Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe’s expansion to accelerate. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

Ah yes. The speed at which the Universe is expanding is not a constant. This is a very interesting consequence of the presence of dark energy in our Universe, and gives us an interesting view into the very distant future of our Universe. 

Dark energy is a descriptive term we have applied to a force which is responsible for the observed expansion of the Universe.  We can see that the Universe is expanding, by measuring the apparent speeds of objects in the Universe, all of which appear to be receding at a rapid clip. Considering that we’re not at a special place in the Universe, this observation is best explained by every object drifting away from every other object. Given that gravity is also present in our Universe, some other force must be acting upon each and every object in our Universe in order to counteract gravity and keep them on their paths to an increasingly isolated Universe. 

This force which counteracts gravity has been dubbed Dark Energy, and exactly what it is and how it operates is still extremely poorly understood. However, based on our observations, it must make up about 68% of all the energy present in the Universe to be able to do what we observe it doing - pushing all galaxies which aren’t tied to each other by gravity further apart from each other.

A representation of the evolution of the universe over 13.77 billion years. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image credit: NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image credit: NASA / WMAP Science Team

It’s one thing to have an expanding Universe. If the expansion occurred at a fixed rate, we would draw our Universe expanding as a straight line in diagrams like the one above. However, we have found that the expansion of our universe is happening at an increasingly rapid pace. Our Universe’s expansion is accelerating, not constant. This means that if I watch two galaxies separating now, from some kind of Universal bird’s eye view, and then came back in a billion years, and watched another set of two galaxies expanding away from each other, the second set would separate much faster than the first set. 

Over a long enough period of time, this increasing speed of expansion means that the density of objects within the Universe will decrease. If every galaxy is increasingly distant from every other galaxy, images of galaxies outside our own Milky Way will also become increasingly out of date, as the light travel time also increases. If we pursue the increasing isolation of galaxies to its logical extreme, we arrive at an end-of-Universe scenario called “heat death”. Heat death arrives when a galaxy runs out of gas to form new stars, and the stars which remain are overwhelmingly either very faint red, brown, and black dwarf stars, black holes, or neutron stars. With no new gas able to arrive into the galaxy, the galaxy must end its star formation. Once the remaining red dwarfs and other stellar objects radiate away the last of their heat, and the entire Universe has reached a single, even temperature, we have arrived at the death of heat in our Universe.  This is currently our Universe’s forecast for its eventual end state - and a direct consequence of having such a large amount of Dark Energy, pressing our Universe outwards into an ever-faster expansion.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

Could We Put A Telescope On The Far Side Of The Moon?

Has anyone thought of building an observatory on the other side of the moon?
LROC WAC color (689 nm, 415 nm, 321 nm) overlain on WAC sunset BW image. Note the proximity of the landing site to a contact between red and blue maria. Image credit: NASA/GSFC/Arizona State University

LROC WAC color (689 nm, 415 nm, 321 nm) overlain on WAC sunset BW image. Note the proximity of the landing site to a contact between red and blue maria. Image credit: NASA/GSFC/Arizona State University

Originally posted on Forbes!

Many times! There is currently a small UV-sensitive telescope on the Moon, which landed there in 2013 as part of the Chinese lander Chang’e 3, and it has taken some interesting images from the Moon to relay back to us, but it’s been placed on the near side of the moon, along with the rover, for easier communication with Earth.

In general, the Moon has been considered an interesting place to put telescopes, because it’s a stable patch of ground, with no atmosphere around to interfere with the telescope. There are a lot of wavelengths of light that can currently only be observed from orbiting space telescopes - ultraviolet is mostly blocked by our atmosphere, as is gamma radiation, and infrared. So, if you can place a UV telescope on the surface of the Moon (as the Chinese lander did), you have a setup that doesn’t need gyroscopes to stabilize the telescope, and it can observe freely without the interference from the Earth's atmosphere.

The Moon isn’t an ideal place to put all telescopes though - your telescopes have to be pretty durable to survive the temperature extremes on the Moon between daylight and shadow. With temperatures swinging from -298F to 224F, this is not a particularly kind place for electronics. A temperature swing of more than four hundred degrees, once every two weeks, is not for the fainthearted.

An Apollo 11 oblique view of the lunar farside in the area of International Astronomical Union crater No. 312, which is about 30 statute miles in diameter. Image credit: NASA.

An Apollo 11 oblique view of the lunar farside in the area of International Astronomical Union crater No. 312, which is about 30 statute miles in diameter. Image credit: NASA.

Optical telescopes on the Moon are similarly tricky - for two weeks out of every month, the telescope would be in daylight, no matter where you put it. The telescope would have to survive two weeks of extremely hot temperatures, cool back down, and then it could observe for a maximum of two weeks. Infrared cameras are an even worse fit; the thermal cameras attached to an infrared telescope are extremely sensitive to heat (by design) and without being able to fully shield the camera from the Sun, as we do for space telescopes, the detectors are likely to be unhappy after a few lunar temperature cycles.

There’s an additional complication when it comes to the far side of the Moon in particular - it’s very hard to communicate with anything on the far side of the Moon. Radio waves can’t reach it, and so orbiting crafts have habitually just had a communications blackout while they’re behind the Moon (from an Earthbound perspective). There’s only one type of observatory for which this is an explicit benefit - the radio.

On Earth, almost everything interferes with the clear observation of an astrophysical source in radio wavelengths. Everything from radar, to cell phones, to microwaves, to GPS satellite communications with the ground, to digital cameras, will interfere with the incoming signal from space. In general, the only solution is to put the radio telescopes very far away from as many of these things as we can, and hope for the best. But the far side of the Moon is shielded from all of this by the entire bulk of the Moon, and would truly be an interference-free area to put a radio telescope.

It isn’t problem free, or we might already have a lunar radio telescope. The temperature stresses are still significant for a radio telescope. One option would be to construct a dish, like Arecibo, inside a lunar crater, but that would be a technical challenge without a more significant human presence on the Moon. However, some radio telescopes can function extremely well with simpler electronics. The LOFAR telescope, for instance, is scattered across Europe, and is mostly made up of a large number of very simple antennas, instead of the complex machine that is the coordination of the dish-style antennas that comprise the Very Large Array in New Mexico. In principle, we could scatter a similar set of simple antennas all over the lunar far side, and create a very large radio telescope.

Photo showing a low-band antenna (LBA) of the Low-Frequency Array (LOFAR), an interferometric radio telescope build in Europe. In the right back of the antenna, a LOFAR cabin is visible that contains electronics. The full array consists of thousands of such antennas. Image credit: A. R. Offringa, CC BY-SA 3.0, via Wikimedia Commons

Photo showing a low-band antenna (LBA) of the Low-Frequency Array (LOFAR), an interferometric radio telescope build in Europe. In the right back of the antenna, a LOFAR cabin is visible that contains electronics. The full array consists of thousands of such antennas. Image credit: A. R. Offringa, CC BY-SA 3.0, via Wikimedia Commons

On the other hand, we still want to get the data back. And transmitting from the far side of the Moon directly, as orbiters can attest, is impossible. So how could we submit the data back home to Earth? Your two options are a heavy-duty cable which extends far enough around the Moon that the Earth would be visible, and if you can feel engineers around the world cringing, don’t worry - that’s not the preferred solution. The better solution is to put a communication satellite in orbit around the Moon, whose primary role would be to communicate between the telescope on the lunar ground, and to Earth, when Earth is again visible from orbit. While these communications may be a source of interference to the telescope, it wouln’t be an issue as long as the telescope stopped trying to see distant objects while its communication tether to the Earth was overhead.

The main challenge now, other than developing a way to deploy a large number of antennas on the Moon, is simply cost. While we have no rocket which could transport humans to the Moon at the moment, we have certainly sent orbiters to the Moon in recent years. It’s expensive, though, and with a NASA budget that’s increasingly constrained, pulling together the funds for a telescope on the far side of the Moon will be the major constraint.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

Haven't The Stars In The Oldest Galaxies Died By Now?

If it has taken 13 or so billion light years for the light from the most distant observable stars to reach Earth, is it not probable that those stars no longer exist but have “gone nova”?

Originally posted on Forbes!

It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million...

It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million years. Even if the light caught in our detectors was generated in that star’s earliest days, by the time that light reached us, 13 billion years later, that star must be long gone. Our own Sun, which has a comparatively lengthy 8 billion year lifespan, wouldn’t have survived for the length of time it takes for the light from the most distant known galaxies to reach us.

What does that mean for the galaxy as we see it? For starters, it means that its population of stars has changed almost entirely since that light began its travels. The brightest stars have all died, exploding out to recycle their gas into their stellar neighborhood, and possibly triggering the formation of a new round of stars. In 13 billion years, this may happen many, many times. But the stellar recycling act isn’t perfect. For every large, extremely bright star that’s formed, we typically expect a number of smaller, fainter stars to also form. These smaller stars (like our Sun) live longer and don’t explode as violently (or at all) at the end of their lifetimes. Stars which are even smaller and redder than our own will live even longer, and may even persist for the entire length of the trip the light took to reach us. This means that over time, the galaxy will build up a reservoir of faint, reddish stars, which limits the amount of gas present in the galaxy available to make new stars. This sequence of events is one of the suggestions for how we end up with the giant elliptical galaxies in the nearby universe -- their population of stars is mostly red, and they seem to have very little gas.

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

But it’s not the only pathway open to that distant galaxy -- it’s also possible to refill the galaxy with gas, allowing the galaxy to continue forming the brightest, bluest, shortest-lived stars for a longer period of time. Depending on whether or not the galaxy finds itself surrounded by smaller, gas-filled galaxies, or on its own in a more lonely part of the Universe, that distant galaxy’s course will change again. If we could sit and watch that distant galaxy’s evolution for a few billion more years, we would be able to say for sure which pathway that galaxy was sent down. I don’t know about you, but I certainly don’t have a hundred million years to wait.

So without millions or billions of years to wait for updates from that galaxy, we’re a bit stuck. We effectively have a snapshot of this earliest galaxy as it was, and no ability to check what it did later, or how the galaxies around it changed with time. What we do have is another snapshot of the Universe later, where a different set of galaxies exist, with the same inability to watch where they go forward in their path through their own lives. Trying to piece together which galaxies in the distant universe might evolve into the galaxies we see at more recent times, at less extreme distances, is one of the fundamental puzzles of observational astronomy.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

If Time Doesn't Exist For Photons, How Does Anything Happen To It?

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Images showing the expansion of the light echo of V838 Monocerotis. Image credit: NASA, ESA, H.E. Bond (STScI) and The Hubble Heritage Team (STScI/AURA)

Originally posted at Forbes!

The concept of photons running with stopped clocks is something that is pulled straight out of relativity; the faster you’re moving, the slower your onboard clocks are moving, and the closer to the speed of light you’re operating, the more sluggish they get. Once you reach the speed of light, your clock runs infinitely slow - for practical purposes, we can say that time doesn't flow for the photon. As with all things relativity, this isn’t an absolute statement- light still has a finite speed, and we can observe light taking fixed amounts of time to traverse large distances.

When light goes zipping around our Universe, it is physically moving through space at a speed of 186,000 miles every second.  But if you could affix a clock to it, an observer that’s not moving at the speed of light would not see the clock moving forwards the way their own clocks do. A hypothetical person moving at the speed of light wouldn’t notice anything weird with their clock, but what they might notice is that the Universe is full of things to smash into.

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

This artist's impression shows how photons from the early universe are deflected by the gravitational lensing effect of massive cosmic structures as they travel across the universe. Image credit: ESA

No matter how fast you’re going, if there’s something in front of you, and you can’t dodge it, you will hit it. This is as true for humans as it is for light, and light is even less capable of dodging an oncoming object than we humans are.  Light always travels in locally straight lines - the only way to bend light is to make a curve in the shape of space. A photon will then follow that curve, but there’s no onboard navigation.

Photons are effectively stuck playing the world’s most obnoxious game of bumper cars, continually bouncing from impact to impact. From our non-speedy perspective, the clocks on photons do not tick forward between impacts, so if the photon has the good fortune to get re-emitted by whatever it ran into, it will, from our viewpoint, instantaneously smash directly into something else without its onboard clock ticking onwards at all.

The photon may not get re-emitted by whatever it ran into, (this is one way to get rid of a photon). The energy of whatever it hit will increase, so the energy isn’t lost. However, if it hits something particularly cold, the object won’t be radiating much, and the photon’s energy will be a convenient donation.  More commonly, after some amount of time, a new photon will be produced, at a different energy level, carrying energy away from whatever the photon punched itself into earlier. That new photon has an equally short apparent flight until it smashes into something else.

It’s not the most glamorous of paths through the Universe, but a continual ricocheting from solid matter to solid matter is how photons in our Universe go about it.

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

The bright cloud is a reflection nebula known as [B77] 63, a cloud of interstellar gas that is reflecting light from the stars embedded within it. There are actually a number of bright stars within [B77] 63. Image credit: ESA

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!

Would A White Dwarf Outlive A Neutron Star?

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Image credit: NASA/DOE/Fermi LAT Collaboration, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF, JPL-Caltech/UCLA

This multiwavelength composite shows the supernova remnant IC 443, also known as the Jellyfish Nebula. Fermi GeV gamma-ray emission is shown in magenta, optical wavelengths as yellow, and infrared data from NASA's Wide-field Infrared Survey Explorer (WISE) mission is shown as blue (3.4 microns), cyan (4.6 microns), green (12 microns) and red (22 microns). Image credit: NASA/DOE/Fermi LAT Collaboration, Tom Bash and John Fox/Adam Block/NOAO/AURA/NSF, JPL-Caltech/UCLA

Originally posted at Forbes!

This is an interesting question, because most of the time when we talk about the lifetime of a star, we mean the part before the creation of a white dwarf or a neutron star.  Typically, a star has a “lifetime” when it is burning hydrogen into helium, gradually consuming its central reservoir of hydrogen, producing light as it does so.

It’s using this kind of definition for “lifetime” that we say our home star Sol is middle aged. Our Sun is a little over 4.5 billion years old, about halfway through its estimated ~9 billion years of hydrogen burning. Once hydrogen burning is complete, our Sun, along with all other stars of its mass, will go through an elaborate end-of-life transformation, which involves shedding most of the outer layers of the star. In the case of the Sun, we’ll produce a planetary nebula out of the discarded outer surface of the star, leaving only the hot nucleus of the star behind as a white dwarf.

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Image credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

This colourful bubble is a planetary nebula called NGC 6818, also known as the Little Gem Nebula. A version of the image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Judy Schmidt. Image credit: ESA/Hubble & NASA; acknowledgement: J. Schmidt (geckzilla.com)

If the star were a bit larger, it would explode as a supernova instead of creating a planetary nebula and white dwarf. The star has to sit within a very specific range of masses in order to create a neutron star - too large, and the gravitational weight of the star can compress the core of the star down into a black hole, instead of stopping at a neutron star. If the star is too light, it won’t supernova at all, and you’re left with a white dwarf instead of a neutron star.

Whether you have a white dwarf or a neutron star, both are considered stellar remnants. They’re what remains of a star, after fusion has stopped in the core of the star, and gravity has dealt with the unstable star. Neither the white dwarf or the neutron star will be generating any new heat in their cores, or will be able to do much else, beyond sit there, without some kind of external influence.

A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core. This object, called the Helix nebula, lies 650 light-years away, in the constellation of Aquarius. Image credit: NASA/JPL-Caltech

A dying star is throwing a cosmic tantrum in this combined image from NASA's Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX). In death, the star's dusty outer layers are unraveling into space, glowing from the intense ultraviolet radiation being pumped out by the hot stellar core. This object, called the Helix nebula, lies 650 light-years away, in the constellation of Aquarius. Image credit: NASA/JPL-Caltech

A white dwarf, on its own, will sit in place and glow, gradually losing energy to the deep dark of space, until the white dwarf has lost enough heat to match the background temperature of the universe, at about 3 degrees Kelvin. If the white dwarf could achieve this, we would call the resulting object a black dwarf, but so far, none have ever been observed. This lack of black dwarfs isn't a surprise - the amount of time a white dwarf is predicted to need in order to get to black dwarf status is longer than the Universe has been around. There quite literally hasn’t been enough time in the entire universe for a white dwarf to cool.

Neutron stars are also high temperature objects, and all they can do is cool, but they come equipped with frankly astoundingly strong magnetic fields, and so their cooling process is a little more involved. We spot many neutron stars in the night skies by watching them beam X-rays away from themselves. Combined with the extremely rapid rotation that a lot of neutron stars have, neutron stars are often picked out by watching them pulse in brightness, like a high energy radiation lighthouse.

Without some kind of external factor coming into play, neither a white dwarf or a neutron star will cease to exist in our universe, so in that regard a neutron star and a white dwarf are tied for longevity. There are external factors you could invoke to dispose of a white dwarf or a neutron star - these usually come by way of a second star, which donates mass to the white dwarf or to the neutron star. If the white dwarf gains enough material from devouring its neighbor star, it can explode in a supernova, detonating itself into oblivion. A neutron star can also gather enough mass to itself so that it collapses into a black hole, imploding inwards on itself. However, there's no reason for the white dwarf to always do this faster than a neutron star, because the key factor here is how quickly the star can steal material from its companion. The speed of the theft depends on the orbit of the companion, not whether or not you have a white dwarf instead of a neutron star.

Without such a stellar donor, both of these stellar remnants will simply hang out in the Universe, slowly losing their heat, until they have matched the background temperature of the Universe.

Have your own question? Feel free to ask! Or submit your questions via the sidebarFacebook, or twitter.

Sign up for the mailing list for updates & news straight to your inbox!