Cosmology is the study of the universe as a whole -- what happened in its past and what will happen in the future. Cosmology is a fast-moving subject at the moment and often in the news. It has many concepts which are hard to grasp; as such, many of the questions that are sent in to us are about cosmology.
The Big Bang
Credit: ESA and the Planck Collaboration
Planck CMB. The anisotropies of the Cosmic microwave background (CMB) as observed by Planck. The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380 000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and galaxies of today.One of the best known theories in cosmology is the Big Bang. This is the idea that our universe started out much hotter and denser than it is now and has been expanding since then. This theory is based on observations of our universe, among which are:
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External galaxies are receding in such a way that their recessional speeds are proportional to the distance they are away from us (this is called Hubble's Law after Edwin Hubble who first noticed it). This observation is explained well by a uniform expansion of the universe. If the universe is expanding, it must have started out very small some time far in the past. It is this point which has been called the beginning of the universe or the "Big Bang."
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When we observe the night sky we see an excess of radiation which is called the CMB radiation (cosmic microwave background radiation). It is a perfect black body with a temperature of 3 Kelvin. Taken with the expansion of the universe, this radiation says that the universe must have been much hotter in the past and also opaque to radiation. It turns out that the CMB radiation fits in perfectly with being from the first photons to escape after the universe became transparent. The universe became transparent for the first time when atoms first formed (in an event known inexplicably as recombination).
Large scale structure of the universe
Credit: VIRGO - Millennium Simulation Project/Springel et al. (2005)/Max-Planck Institute for Astrophysics
Simulation of dark matter distribution. The Millennium Run used more than 10 billion particles to trace the evolution of the matter distribution in a cubic region of the Universe over 2 billion light-years on a side. By applying sophisticated modelling techniques to the 25 Tbytes of stored output, Virgo scientists have been able to recreate evolutionary histories both for the 20 million or so galaxies which populate this enormous volume and for the supermassive black holes which occasionally power quasars at their hearts. This image shows the dark matter distributions at redshift z=0 (t = 13.6 Gyr).
Galaxies are not randomly scattered across the universe, but rather they are preferentially found in groups or clusters, and the groups and clusters are also seen to be arranged in superclusters. Our own galaxy, the Milky Way, is known to be a member of the group of galaxies that we call the Local Group. The group has two large galaxies (the Milky Way and Andromeda) as well as a host of dwarf galaxies. Clusters are just much larger versions of groups. A good analogy is that the groups are like towns, while clusters are like cities. In this analogy the superclusters are like the state (or country for those of us not from the US) that the towns and cities are in.
Superclusters are not relaxed systems - that means that they are still expanding with the universe, while groups and clusters are (in general) believed to be held together by the mutual gravity of the galaxies in them and so are no longer expanding with the universe. Superclusters form a filamentary network in space, sometimes referred to as a "cosmic web".
By studying this large scale structure in the universe astronomers can learn a lot about cosmology. The superclusters are probably the remnants of the first structure formation in our universe (which you can see above in the image of the CMB) and the way structure first formed depends a lot on the cosmology of our universe.
Cosmology in the 21st Century
As the last century came to a close and the first few years of this century passed us by, cosmologists have been beginning to feel like they are getting close to knowing what the basic parameters of the universe are. A recent talk by a cosmologist at Cornell started with the question "Is cosmology complete?" The answer, by the toss of a coin was no! The talk then went on to explain why in some sense the answer is no, and in others it is yes.
Recent observations (in particular from WMAP) have solved several long-standing problems in cosmology. Evidence strongly suggests that the Hubble Constant is around 71 km/s/Mpc (accurate to about 5 percent) and that the universe is geometrically flat, but that it is probably dominated by some weird form of energy called "dark energy." "Ordinary" matter also seems to be dominated by "dark matter," which cannot be the same as the matter (called baryonic matter) that makes up humans. Overall, it's a strange universe, but one that we are beginning to understand in greater detail than ever before!
Questions About Cosmology and the Big Bang
- What was there before the Big Bang and what is there outside of our universe? (Beginner)
- What makes up most of the Universe? (Beginner)
- How is it that we see farther out in space (farther back in time) than in the past? (Beginner)
- Can I calculate the size of the Universe by converting light years to kilometres? (Intermediate)
- Where did all the water in the universe come from? (Intermediate)
- What is the largest structure in the universe? (Intermediate)
- What is the mass of the Universe? (Intermediate)
- Why isn't the sky bright at night if the universe has so many stars? (Intermediate)
- Is light blueshifted when it is gravitationally lensed? (Intermediate)
- Why do we assume that the universe is homogeneous? (Intermediate)
- What do "homogeneity" and "isotropy" mean? (Intermediate)
- Can you use an infinite line of reflectors to send light to the edge of the Universe? (Intermediate)
- Where did the Universe come from? (Intermediate)
- Did the speed of light change over the history of the universe? (Intermediate)
- Where did the matter in the universe come from? (Intermediate)
- How can we compare dark matter and dark energy? (Intermediate)
- What are the possible ends of the universe? (Intermediate)
- What is the difference between homogeneity and isotropy? (Advanced)
- How will a change in the speed of light affect the evolution of the Universe? (Advanced)
- How is the Cepheid yardstick validated? (Advanced)
- How do we know that superclusters are the largest structures in the universe? (Advanced)
- How are light and heavy elements formed? (Advanced)
- How do we determine the age of the universe? (Advanced)
- What caused the Big Bang and created the Universe? (Beginner)
- Can we look back far enough in time to see the Big Bang? (Beginner)
- Have there been other Big Bangs before? (Beginner)
- How has light from 13 billion years ago not passed us by? (Beginner)
- If the Universe was infinitely dense at the Big Bang, why didn't time stand still? (Beginner)
- Can we find the place where the Big Bang happened? (Intermediate)
- Could there have been multiple Big Bangs at different places in the universe? (Intermediate)
- Wouldn't the Big Bang theory be considered a hypothesis and not a theory? (Intermediate)
- When we look back to the Big Bang, why don't we see the universe as a tiny speck of matter? (Intermediate)
- Where was the water in the big bang? (Intermediate)
- Did time go slower just after the Big Bang? (Advanced)
- If spacetime is curved, is it possible for us to look into space and see light from Earth from long ago? (Intermediate)
- How can the universe be "flat"? We're 3D! (Beginner)
- What is a dimension? (Intermediate)
- What is the shape of the universe? (Intermediate)
- How can geometry be different in space? (Intermediate)
- What happens to spacetime inside a black hole? (Intermediate)
- Why is the Universe flat and not spherical? (Advanced)
- What would an "open geometry" for the universe look like? (Advanced)
- What are "curled" dimensions? (Advanced)
- How do we know what we observe is x light years away? (Beginner)
- Our universe is still expanding, does that mean things in our daily life are expanding? (Intermediate)
- Can two galaxies move away from each other faster than the speed of light? (Intermediate)
- Why is it difficult to understand the expansion of the universe? (Beginner)
- How do we know if the universe will keep expanding forever? (Intermediate)
- Could the Universe have expanded faster than the speed of light at the Big Bang? (Intermediate)
- How is it proved that the Universe is expanding? (Intermediate)
- Can the acceleration of the Universe be analogous to apparent acceleration in a airplane? (Intermediate)
- As the universe expands, why don't galaxies get stretched out? (Intermediate)
- What is the universe expanding into? (Intermediate)
- Could our position in the universe affect our perceptions of the expansion? (Intermediate)
- How fast is the Universe expanding? (Intermediate)
- How do we define distance in an expanding universe? (Intermediate)
- How are galaxy distances inferred from their recessional velocities? (Intermediate)
- No matter how fast the universe is expanding, shouldn't gravity eventually make it recollapse? (Intermediate)
- Is the universe expanding faster than the speed of light? (Intermediate)
- Is the universe really like an expanding balloon? (Intermediate)
- Is dark energy affected by black holes? (Intermediate)
- When measuring the expansion of the universe, do astronomers consider that they're seeing how galaxies moved long ago, not today? (Intermediate)
- Cosmologists say the expansion of the universe is accelerating, what is driving this? (Intermediate)
- Is cosmic expansion happening on microscopic scales? (Advanced)
- What is the difference between the "Doppler" redshift and the "gravitational" or "cosmological" redshift? (Advanced)
- What would the Big Crunch look like to an observer on Earth? (Advanced)
- If the universe is "bounded" today, could it one day become "unbounded"? (Advanced)
- How can observations of the distant universe prove that the expansion is accelerating *now*? (Advanced)
- How do supernovae show us that the Universe's expansion is accelerating? (Advanced)
- What is the most distant known galaxy? (Beginner)
- How big is the Universe? (Intermediate)
- How do we know the age of the Universe and the Earth? (Intermediate)
- Where, in relation to the entire universe, is the Milky Way located? (Intermediate)
- Is the Earth at the centre of the Universe? (Intermediate)
- What is at the end of the Universe? (Intermediate)
- How does the Earth compare in size to the entire universe at the present epoch? (Intermediate)
- If the universe is infinite does that mean there is an infinite number of "me"s? (Intermediate)
- If clocks run slow in a gravitational field, how can we know the true age of the Solar System and Universe? (Intermediate)
- How big is the Hubble Ultra Deep Field image? (Intermediate)
- If there are infinity universes, will every thing that is possible happen? (Intermediate)
- If the universe is 13.8 billion years old, how can it be larger than 13.8 billion light years across? (Advanced)
- What is a singularity? (Beginner)
- What does the term "visible universe" mean? (Beginner)
- What is dark matter? (Beginner)
- What is "nothing", from a scientific point of view? (Beginner)
- Wait, I'm still confused why information can't travel faster than the speed of light. (Beginner)
- What is a dimension? (Intermediate)
- Wouldn't the Big Bang theory be considered a hypothesis and not a theory? (Intermediate)
- How do we define distance in an expanding universe? (Intermediate)
- What do "homogeneity" and "isotropy" mean? (Intermediate)
- What are "curled" dimensions? (Advanced)
- What is dark matter? (Beginner)
- Could photons be dark matter? (Intermediate)
- Could a different theory of gravity explain the dark matter mystery? (Intermediate)
- What's the difference between dark matter and dark energy? (Intermediate)
- How can we compare dark matter and dark energy? (Intermediate)
- What is Hot Dark Matter theory? (Intermediate)
- Could neutrinos be dark matter? (Intermediate)
- Could the Universe's dark matter be made up of black holes? (Advanced)
- What effect did compression waves have immediately following the Big Bang? (Intermediate)
- How has light from 13 billion years ago not passed us by? (Beginner)
- Wait, I'm still confused why information can't travel faster than the speed of light. (Beginner)
- How can the Universe expand faster than the speed of light during inflation? (Advanced)
- Does inflation allow for an infinite number of universes? (Advanced)
- Can "tired light theory" explain the observed redshifts of galaxies? (Intermediate)
- Could matter-anti-matter repulsion power the expansion of the universe? (Intermediate)
- Could a different theory of gravity explain the dark matter mystery? (Intermediate)
- Could dark energy come from matter anti-matter pairs forming spontaneously and then annihilating? (Advanced)
- Do galaxies that are receding from us faster than the speed of light disappear from our observations? (Intermediate)
- How could galaxies have gotten so far away in only 14 billion years? (Intermediate)
- Why are there blue shifted galaxies? (Intermediate)
- How are galaxy distances inferred from their recessional velocities? (Intermediate)
- As the universe expands, why don't galaxies get stretched out? (Intermediate)
- How do galaxies collide in an expanding universe? (Intermediate)
- Is the Local Group expanding along with the entire Universe? (Intermediate)
- When measuring the expansion of the universe, do astronomers consider that they're seeing how galaxies moved long ago, not today? (Intermediate)
- How do distant galaxies differ from those nearby? (Advanced)
- Will the diameter of a cluster of galaxies change with redshift? (Advanced)
- Why does the apparent density of galaxies drop off at larger distances? (Advanced)
The Ask an Astronomer team's favorite links about Cosmology and the Big Bang:
- Scale of the Universe Interactive illustration of the scales of the Universe
- NASA Intro to Cosmology Page
- Ned Wright's Cosmology Tutorial: Comprehensive discussion of cosmology from a UCLA professor, including news and frequently asked questions (in English, French and Italian).
- Cambridge Cosmology Public Site
- Discussion of the concept of infinity in cosmology, from Professor John D. Barrow of the University of Cambridge
How to ask a question?
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