When we talk about the age of the universe, we usually mean the age of theobservable universe. All of our observations occur from Earth or its near vicinity.
In the above picture, imagine that Earth is the circle in the center. We look outwards and the farthest we can see is a radius (d) that equals the age of the universe (a) times the speed of light (v). So, if we see something 13.8 billion light years away, the universe must be at least 13.8 billion years old.
What complicates this is that we believe the universe is not a static object. It is expanding. The big bang theory tells us that the universe started as an extremely dense and compact object and has been expanding outwards, ever since the big bang. It is also cooling.
Early attempts to determine the age of the universe focused on redshift.
Redshift
The Doppler effect tells us that if an object is moving towards us, the waves it emits are compressed and if it is moving away from us, the waves are elongated. You may notice this when a police car or ambulance passes you, that the siren sounds different as the vehicle is approaching you than when it is moving away. The same thing happens with color.
The visible light spectrum looks like this:
So, if an object is approaching, the light it is emitting will be shifted slightly in the direction of blue. If the object is moving away, it will be shifted slightly in the direction of red.
If we start with the premise that the big bang happened - that the universe, at the beginning, was infinitesimally small and has been expanding ever since, we can imagine that the galaxies are all moving away from each other. The word choice here can get a little finicky. It isn't necessarily so much that the galaxies are being propelled away, but space is expanding, between those galaxies, so they are becoming farther apart.
That means if we look at a very far away galaxy, it should appear more red than we would expect it to be. We can analyze the components of a galaxy and based on its size, mass, temperature and activity determine what color it should naturally be. The difference between the color it should be, and the color it appears to us is called the redshift. If we correctly understand how fast space expands, we can derive the distance between the two objects. That rate of expansion is called the Hubble Constant (H0). H0 = v/d where v is the radial velocity of the galaxy we're observing and d is the distance from Earth.
If we determine the distance to that far away galaxy (d) and we know that the speed of light (c) is constant, then we know how long it took that light to reach us. The universe cannot be younger than that amount of time. So, based on our current understanding of the Hubble constant and the farthest away galaxies we've been able to observe, we know that the universe has to be at least 13.8 billion years old for us to be looking at those galaxies.
Cosmic Microwave Background Radiation
More recently, observations have focused on the cosmic microwave background radiation. Cosmologists modeling the big bang believe that for the first ~370,000 years after the big bang, the universe was too dense for photons to be emitted. But at around 370,000 years, it had expanded enough that photons were emitted that described the state of the universe at that time via a very specific ripple pattern, from the combination of electrons and protons combining to produce the first hydrogen. This radiation has also been redshifted.
While mapping this cosmic background radiation, observers have detected variations called anisotropies. These variations are expected because of the expansion of the universe - in fact, study of the anisotropies can reveal how much the universe has expanded and thus tell us how old the observable universe is.
Over the last decade, data from WMAP and now Planck (two observatories out in space) have given us a very detailed map that tells us, if we understand the physics properly, that the universe is about 13.8 billion years old.
In the above picture, imagine that Earth is the circle in the center. We look outwards and the farthest we can see is a radius (d) that equals the age of the universe (a) times the speed of light (v). So, if we see something 13.8 billion light years away, the universe must be at least 13.8 billion years old.
What complicates this is that we believe the universe is not a static object. It is expanding. The big bang theory tells us that the universe started as an extremely dense and compact object and has been expanding outwards, ever since the big bang. It is also cooling.
Early attempts to determine the age of the universe focused on redshift.
Redshift
The Doppler effect tells us that if an object is moving towards us, the waves it emits are compressed and if it is moving away from us, the waves are elongated. You may notice this when a police car or ambulance passes you, that the siren sounds different as the vehicle is approaching you than when it is moving away. The same thing happens with color.
The visible light spectrum looks like this:
So, if an object is approaching, the light it is emitting will be shifted slightly in the direction of blue. If the object is moving away, it will be shifted slightly in the direction of red.
If we start with the premise that the big bang happened - that the universe, at the beginning, was infinitesimally small and has been expanding ever since, we can imagine that the galaxies are all moving away from each other. The word choice here can get a little finicky. It isn't necessarily so much that the galaxies are being propelled away, but space is expanding, between those galaxies, so they are becoming farther apart.
That means if we look at a very far away galaxy, it should appear more red than we would expect it to be. We can analyze the components of a galaxy and based on its size, mass, temperature and activity determine what color it should naturally be. The difference between the color it should be, and the color it appears to us is called the redshift. If we correctly understand how fast space expands, we can derive the distance between the two objects. That rate of expansion is called the Hubble Constant (H0). H0 = v/d where v is the radial velocity of the galaxy we're observing and d is the distance from Earth.
If we determine the distance to that far away galaxy (d) and we know that the speed of light (c) is constant, then we know how long it took that light to reach us. The universe cannot be younger than that amount of time. So, based on our current understanding of the Hubble constant and the farthest away galaxies we've been able to observe, we know that the universe has to be at least 13.8 billion years old for us to be looking at those galaxies.
Cosmic Microwave Background Radiation
More recently, observations have focused on the cosmic microwave background radiation. Cosmologists modeling the big bang believe that for the first ~370,000 years after the big bang, the universe was too dense for photons to be emitted. But at around 370,000 years, it had expanded enough that photons were emitted that described the state of the universe at that time via a very specific ripple pattern, from the combination of electrons and protons combining to produce the first hydrogen. This radiation has also been redshifted.
While mapping this cosmic background radiation, observers have detected variations called anisotropies. These variations are expected because of the expansion of the universe - in fact, study of the anisotropies can reveal how much the universe has expanded and thus tell us how old the observable universe is.
Over the last decade, data from WMAP and now Planck (two observatories out in space) have given us a very detailed map that tells us, if we understand the physics properly, that the universe is about 13.8 billion years old.
No comments:
Post a Comment