The Cosmos
Questions about the Universe have intrigued us since we first
gazed up at the sky. Where do we fit in this vast expanse, how
did it all begin, and are we the only ones “out there”? In this
issue of Search + Discover, University of Minnesota faculty
address such questions as:
Explore further reading and related links on the subjects of physics and astronomy.
Light takes some 100,000 light years to travel from one end of our galaxy to the other. So, when we look at the stars, aren’t we looking back in time?
The answer is a very simple—yes!
For example, the light we receive from the sun left it eight minutes ago. The finite velocity of light provides us with a time machine that lets us look at the past but not the future. We do not see the stars or the distant galaxies as they are now, but as they were when the light left them.
—Roberta Humphreys, professor and associate dean, Department of Astronomy, Twin Cities campus
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Why does the moon look so huge when it is rising?
To the human eye, a full moon looks bigger on the horizon than it does high in the sky. This is a famous problem known as the “Moon Illusion,” that actually applies to the sun as well. This effect really is an optical illusion. If you take two pictures of a full moon, at the horizon and higher in the sky, you will see that they are the same size in the photographs, but to your eye, the moon at the horizon appears larger. Another way to convince yourself it is actually an illusion is if you hold your thumb up to a full moon to measure its size. You will see that a full moon is about as wide as your thumb all the time, no matter where it is in the sky. By doing this you have shown that the angular size of the moon is always the same, about 0.5 degrees.
The reason why a full moon appears larger at the horizon is still being debated.
A commonly proposed theory suggests that the reason the moon appears bigger at the horizon is because of the way humans perceive the geometry of the sky. At the horizon, the sky appears more “curved” which may serve to skew how our brains perceive sizes and distances. This would make the moon seem larger compared to when it is high in the sky where the sky appears more “flat.”
There has been a lot of research into the “Moon Illusion” but there is not yet a definitive explanation. The only thing we can be sure of is that it actually is an illusion.
—Daniel Weisz, Ph.D. candidate, Department of Astronomy, Twin Cities campus
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How can outer space be infinite? Doesn’t it have to ‘end’ somewhere?
Space may be infinite or finite. All we can be sure of is that it is much larger than the observable Universe—that is, the part of the Universe we can see. The most distant objects we can see in the Universe are at what is called the horizon, about 41 billion light years away. As time goes on, this horizon will expand and we will see more of the Universe.
The space in our Universe may be curved. If space is positively curved like a sphere, the Universe would indeed be finite, though it may be unimaginably large. In that case, light—or spaceships—would be forced to travel along the curve of space, just as cars and trains must travel along the curve of Earth’s surface. Eventually, the light or the spaceship would end up where it started.
But space may be flat, like a plane. Light or spaceships traveling in a flat plane could keep going forever, and space would be infinite. If space is negatively curved, then light or objects would still travel along curved lines, but ones that never led back to the point of origin, and space would again be infinite. In fact, space appears to be close to flat, but whether it’s completely flat or curved a bit isn’t known.
—Keith Olive, Distinguished McKnight University Professor, Department of Astronomy, Twin Cities campus
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Why are all planets and stars spherical?
In everyday life, we’re used to looking at things that have a really wide variety of shapes, from rounded pebbles, to jagged rocks, flattened or puffy clouds, or the complex shapes of living things.
Planets and stars, by contrast, only take nearly spherical shapes.
Why the difference? It’s because of the different physical forces that give each object its structure. All materials are shaped to some extent by the chemical forces between atoms and molecules.
These chemical forces are slightly different for each material, and they are what give everyday objects their wide range of structural properties. Another force that works to hold things together is gravity, but for familiar-sized objects, the gravitational attraction is tiny. Planets and stars, however, are many billions of times more massive, and so the chemical forces between molecules are overwhelmed by the force of gravity. This is because the more massive the object, the stronger the force of its gravity. Gravity pulls every part of a planet or star down towards its center, overcoming the structural strength of the material making up the object, and pulling it down into the smallest possible volume, which is a sphere. Stars, which are made out of very hot gases, lack any solid material that could form any non-spherical structures, so they are even more perfectly spherical than planets.
—Andrew Cole, postdoctoral researcher, Department of Astronomy, Twin Cities campus
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What was going on around here before the Big Bang?
This oft-posed question is somewhat nonsensical to Big Bang theorists, says Big Bang expert Keith Olive. That’s because, according to their scientific analysis, the Big Bang was the event that created both space and time. Therefore, there was no “around here” and no “before” until the Big Bang occurred.
For people who may have a hard time wrapping their brain around that concept, Olive provides this analogy:
“Imagine, that instead of being three dimensional, space is a two-dimensional surface, and let’s think of that surface as a balloon. Now, let’s think of the radius of the balloon as time. As I blow it up, the surface of the balloon gets bigger. And if I’m watching it as a movie, I’m seeing the balloon at different stages in time where its radius is bigger. That balloon is the universe and the Big Bang represents the appearance of the balloon and the beginning of time.”
Olive acknowledges that some may find these cosmic concepts unfathomable.
“I think it’s hard for people to imagine the space being created, let alone time being created,” he says. “You can imagine stuff appearing in space at a certain time. That’s what many people imagine: the universe was there, time was going on and then all of a sudden at 5 o’clock was a big explosion and all this matter came out. But that’s not what the Big Bang is. The Big Bang is actually the creation of the space and of the time.”
—For more information about Olive’s research, see the link below to the article “Searching for clues to the early universe.”
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What would happen if a sufficiently large asteroid or comet hit the moon and obliterated it or altered its orbit?
The answer depends on just how big “very large” is. Let’s take Ceres, the largest asteroid in the Solar System. If Ceres hit the moon, it would indeed be catastrophic, even though Ceres, roughly the size of Texas, is only about 1.3 percent the mass of the moon. If Ceres was traveling at typical orbital speeds in the inner Solar System, say 30 km/sec (about 67,000 mph) and hit the moon, enough energy would be released to melt a significant fraction of the moon and dramatically disrupt the moon’s orbit. In addition, large amounts of debris would fly off into space, some of it hitting the Earth with disastrous consequences for life there.
Ceres, along with the other large asteroids, is well constrained in its orbit around the sun and is extremely unlikely ever wander away and hit the moon. The asteroid that hit the Earth 65 million years ago at the end of the Cretaceous, and probably caused the extinction of the dinosaurs, was much smaller than Ceres, about the size of the city of St. Paul. If an object this size hit the Moon traveling at typical orbital speeds, it would make a spectacular explosion and leave behind an impressive crater, but it would not significantly alter the orbit of the Moon.
Comets are rarely very large, typically only a few kilometers across. If a good-sized comet hit the moon (and many must have done so over the history of the Solar System), it would be similar to the Cretaceous impact. There would be a spectacular blast, leaving a large crater, but not much else. However, some comets have their origin in the Kuiper Belt, which does contain several very large, icy worlds. If a large Kuiper Belt object such as Pluto or Eris ever wandered into the inner Solar System and hit the moon, it would be even more catastrophic than an impact with Ceres. Again, the odds of this happening are extremely remote. The Solar System contains lots of small objects, but very few large objects, making an impact from a very large asteroid or comet extremely unlikely. Remember, the Earth gets hit with 100 million tons of space debris every year, but this is almost entirely made up of small bits of dirt and dust.
—Terry Jones, professor, Department of Astronomy, Twin Cities campus
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What are the chances of intelligent life in outer space?
The odds are “definitely not zero” and are potentially quite high, according to University astronomy professor Charles “Chick” Woodward. Additionally, the odds are on the rise, he says, as scientists apply new information to an equation developed in the 1960s to answer just this question. In 1961, scientist Frank Drake developed The Drake Equation to try to quantify the number of planets in our galaxy capable of producing intelligent life. The equation takes into account factors such as the number of stars in the Milky Way, the fraction of stars that have planets in orbit around them and the number of planets per star that may be capable of evolving intelligent life. At the time, the exercise was largely conjecture, Woodward says. But using sophisticated new telescopes and research methodologies, scientists are increasingly able to plug real numbers into the equation. “Certainly we’re right on the cusp of being able to detect earth mass type planets,” Woodward says. “I think once we do that then the probability begins to go up enormously.” But what are the odds that E.T. may be more science than fiction? “I think the way to look at it is the odds are certainly not zero any more,” he says. “That is intriguing because I would consider our own galaxy to be a modest-sized galaxy, and to quote Carl Sagan, there are ‘billions and billions’ of galaxies out there, so even if the probability is 1 percent of 10 to the 9th power, you’ve got a big number.”
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Last modified on June 13, 2008