The Giant Arc is a large-scale structure that spans 3.3 billion light-years. The Giant Arc comprises galaxies, galactic clusters, and gas. The Giant Arc and other giant systems call into question the homogeneity of the universe. We now know that the universe is not homogeneous. The gas pressure and temperature within these mega-structures are constantly changing. At the core, clusters of neutron stars abound. These gigantic structures are rare.
Great streams of matter blow in on the solar winds. Then, the gravity of the Giant Arc keeps the different matter within the Great Arc.
Our universe isn’t an isotropic (no direction home) universe. The same in all directions is a relative thing. We don’t know what lies outside our observable universe or beyond that. The velocities of galaxies differ—space-time curves around the matter. Each region varies by the amount of matter (and the resulting bending of space-time). The energy within space pushes galaxies apart faster than gravity can hold them in place. Also, the overall expansion rate grows because the emptier parts of the universe move more quickly than the dense parts. The universe is expanding, with every galaxy beyond the Local Group speeding away from us.
All galaxies currently beyond 18 billion light-years are forever unreachable by us, no matter how much time passes. From our vantage point, we observe up to 46.1 billion light-years away. In this universe, the energy density is decreasing.
The universe’s density refers to the amount of matter within a volume of space. Is the universe closed, open, or flat? It depends on who you ask.
If the universe’s density is significant enough for its gravity to overcome the force of expansion, then the universe will curl into a ball.
If the universe’s density is low and unable to stop the expansion, space will warp in the opposite direction. This would form an open universe with negative curvature resembling a saddle.
However, most scientists believe the universe expands in every direction without curving positively or negatively. It is flat.
We can see 93 billion light-years back. If there is more to see, its light hasn’t arrived yet. Or, the reason no one has witnessed the epoch of galaxy formation is that the ancient starlight, after traveling to us through the expanding fabric of space for so many billions of years, has become stretched. The earlier ultraviolet and visible light wavelengths have stretched out to become infrared radiation. An electromagnetic wave’s amplitude (or height) is proportional to its intensity. Infrared waves have a wavelength 1,000x longer than ultraviolet waves.
Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared energy can also reveal objects in the universe that cannot be seen in visible light using optical telescopes.
Beyond distances of ~14.5 billion light-years, space’s expansion pushes galaxies away faster than light can travel.
Dark energy, inherent to space itself, never decreases, even as the universe expands.
As long as the light from any galaxy emitted at the start of the hot Big Bang 13.8 billion years ago would have reached us today, that object is within our presently observable universe. However, not every visual thing is reachable. Our visible universe contains an estimated ~2 trillion galaxies.
As the universe expands, the space between all unbound objects increases.
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding universe. Note that the things start closer than the time it takes light to travel between them, the light redshifts because of the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. Beyond distances of ~14.5 billion light-years, space’s expansion pushes galaxies away faster than light can travel. Looking back through cosmic time in the Hubble Ultra Deep Field, ALMA traced carbon monoxide gas. This enabled astronomers to create a 3-D image of the star-forming potential of the cosmos. The shared fates of the universe (top three illustrations) all correspond to a universe where the matter and energy fight against the initial expansion rate. In our observed universe, some dark energy, hitherto unexplained, causes a cosmic acceleration.
Dark energy, inherent to space itself, never decreases, even as the universe expands.
How matter (top), radiation (middle), and a cosmological constant (bottom) all grow with time in an expanding universe. As the universe expands, the matter density dilutes, but radiation becomes cooler than its wavelengths get stretched to more extended, less energetic states. Dark energy density will genuinely remain constant. All galaxies beyond a certain distance always remain unreachable, even at the speed of light. Our deepest galaxy surveys can reveal objects tens of billions of light-years away, but there are more galaxies within the observable universe we still have yet to disclose. There are parts of the universe that are not yet visible today that will someday become observable to us, and there are parts that are visible to us that are no longer reachable by us, even if we travel at the speed of light. The size of our observable universe (yellow) and the amount we can reach (magenta). The limit of the visible universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. However, beyond about 18 billion light-years, we can never access a galaxy even if we traveled towards it at the speed of light. Given enough time, light emitted by a distant object will arrive at our eyes, even in an expanding universe. However, if a distant galaxy’s recession speed reaches and remains above the speed of light, we can never contact it, even if we can receive light from its distant past. Only 6% of observable galaxies remain reachable; 94% already lie beyond our reach.
Located a mere 3.6 Megaparsecs away from our Local Group, the M81 group is the nearest substantial group of galaxies to our own Local Group but will remain gravitationally unbound. After that, only our Local Group will stay within reach. Andromeda dominates the Local Group of galaxies and the Milky Way and additionally comprises about 60 other, smaller galaxies. All are within ~5 million light-years of one another, with the nearest galactic groups beyond our own remaining gravitationally unbound from ourselves for all time.
