The Known Universe
Distance to the Edge of the Observable Universe

The Known Universe
Distance to the Edge of the Observable Universe
It is commonly theorized that the universe began with the Big Bang 13.7 billion years ago. But since we can only see as far as light has traveled in that time, we can't actually make out the edge of the universe. Could it be that the universe is infinite? Is there any way to find out what the shape of the universe really is? Can we find the edge, discover what might lie beyond it, and perhaps even discover a universe next to ours?

The age of the universe is about 13.75 billion years, but due to the expansion of space we are now observing objects that are now considerably farther away than a static 13.75 billion light-years distance.

The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years), putting the edge of the observable universe at about 46–47 billion light-years away.

The particle horizon is the maximum distance from which particles could have traveled to the observer in the age of the universe.

It represents the boundary between the observable and the unobservable regions of the universe, so its distance at the present epoch defines the size of the observable universe.

The existence, properties, and significance of a cosmological horizon depend on the particular cosmological model being discussed.

The Universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space.

In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion.

Assuming the Universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction—that is, the observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the Universe as a whole.

The actual shape of the Universe may or may not be spherical.

However, the portion of it that we (humans, from the perspective of planet Earth) are able to observe is determined by whether or not the light and other signals originating from distant objects has had time to arrive at our point of observation (planet Earth).

Therefore, the observable universe appears from our perspective to be spherical.

Every location in the Universe has its own observable universe which may or may not overlap with the one centered around the Earth.

The word observable used in this sense does not depend on whether modern technology actually permits detection of radiation from an object in this region (or indeed on whether there is any radiation to detect).

It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth.

In practice, we can see objects only as far as the surface of last scattering, before which the Universe was opaque to photons.

However, it may be possible in the future to observe the still older neutrino background, or even more distant events via gravitational waves (which also move at the speed of light).

The shape or geometry of the universe includes both local geometry in the observable universe and global geometry, which we may or may not be able to measure.

Shape can refer to curvature and topology. More formally, the subject in practice investigates which 3-manifold corresponds to the spatial section in comoving coordinates of the four-dimensional space-time of the universe.

Cosmologists normally work with a given space-like slice of spacetime called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the backward light cone (points within the cosmic light horizon, given time to reach a given observer).

If the observable universe is smaller than the entire universe (in some models it is many orders of magnitude smaller), one cannot determine the global structure by observation: one is limited to a small patch.

Among the Friedmann–Lemaître–Robertson–Walker (FLRW) models, the presently most popular shape of the Universe found to fit observational data according to cosmologists is the infinite flat model, while other FLRW models include the Poincaré dodecahedral space and the Picard horn.

The data fit by these FLRW models of space especially include the Wilkinson Microwave Anisotropy Probe (WMAP) maps of cosmic background radiation. NASA released the first WMAP cosmic background radiation data in February 2003.

In 2009 the Planck observatory was launched to observe the microwave background at higher resolution than WMAP, possibly providing more information on the shape of the Universe.

Shape of the Universe

What is the shape of our universe? This topic has been debated for quite some time now and unfortunately there is no positive answer. It is believed that there are three possible answers: flat, saddle-shaped or spherical.

It's ironic that not too long ago many people discussed the shape of our planet. At one time it was widely accept that Earth was flat. We now know this is not true. We now question the shape of our universe.

Currently as of 2011, the most widely accepted shape of the universe is that of a flat universe, although this accepted model of the universe may change over time. It is still quite possible that our universe is actually spherical.

It was believed by many scientists that the universe may collapse into a 'Big Crunch' over time, but new evidence appears to point to something completely different. It is now believed that the universe will continue to expand forever because of dark energy.

The nature of this dark energy is a matter of speculation. The evidence for dark energy is only indirect coming from distance measurements and their relation to redshift. It is thought to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity.

Since it is not very dense—roughly 10−29 grams per cubic centimeter—it is hard to imagine experiments to detect it in the laboratory. Dark energy can only have such a profound impact on the universe, making up 74% of universal density, because it uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological constant. Both models include the common characteristic that dark energy must have negative pressure.

Recent results from the Hubble Space Telescope Higher-Z Team indicate that dark energy has been present for at least 9 billion years and during the period preceding cosmic acceleration.

Hunting the Edge of Space

NOVA celebrates the 20th anniversary of the Hubble Space Telescope with a comprehensive look at how a simple instrument, the telescope, has fundamentally changed our understanding of our place in the universe.

Hunting the Edge of Space takes viewers on a global adventure of discovery, dramatizing the innovations in technology and the achievements in science that have marked the rich history of the telescope.

Then NOVA turns its attention to a new generation of ever-larger telescopes, poised to reveal answers to longstanding questions about our universe and, in turn, to raise new questions.

If the Universe is contained within an ever expanding sphere (which may have started from a single point), it can still appear infinite for all practical purposes. Because of length contraction the galaxies further away, which are traveling away from the observer the fastest, will appear smaller.

In this way an infinite Universe fits within a finite sphere as long as the sphere is expanding continually. The question of whether the Universe is infinite can depend on the coordinate system used. For example, you could choose a coordinate system in which the galaxies are equally spaced out and don't have length contraction, in which case the Universe could be said to be infinite in size.

Whichever galaxy the observer is on, the other galaxies moving away from it will appear length contracted. An observer can never get to the edge of the Universe if it is expanding at the speed of light. At the edge of the sphere matter becomes infinitely dense, but because it is moving away from the observer close to the speed of light due to time dilation its effect on the rest of the Universe is negligible. As the spherical Universe expands, matter that was near the edge is now in the middle of the sphere.