Question How do we know that the universe started with a Big Bang?

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If the Big Bang di happen.
Where did it happen?
How old is the Universe?
What is the process from nothing to Now?
No guarantee that nothing exists but if it does nothing could have had a negative energy in it's occupation of infinity.
Or it could have no potential energy and just be unstable.

Either way it could spawn quantum fluctuation that itself is an energy balance and creator of sub atomic particles.
At present day particles are created then destroyed happening at time scales that make our perception look very very slow.

Easy to imagine that if we have a nothing to fluctuation step.
A fluctuation to sub atomic creation step.
A balance of energy for fluctuation that sub atomic particle creation is halted or balanced.
Then a universe from nothing isn't to crazy an idea.

Age of the BB/ this occultation of it is 16+ billion years.
It happened everywhere since it is everything or at least everything in this BB universe.
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Jul 27, 2021
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It's not possible to know where it happened as space is expanding in every direction from every place.
It's estimated to be around 13.7 billion years.
I don't really understand this question.
The thing to keep in mind with the Big Bang and the expansion of the universe is that it wasn’t an “explosion” like a detonation here on earth, with a definite center, and the universe spooling outwards into a pre-existing space.
- Not the whole universe is observed.
- Even from what we are able to observe now. If you take every point in space that exists now, and trace it backwards, all those points get closer and closer together until they reach a mathematical singularity. The Big Bang is, quite literally, everywhere, because the singularity would contain all energy, mass and space.
- There is quite a lot of evidence pointing to the idea of a very tiny universe at the very beginning of our universe; one of the more important being the detection of the cosmic microwave background (CMB). CMB is a fundamental glow in the microwave,any other observations you’re making at this wavelength will be in addition to the CMB.
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The universe is not expanding.
Although we do have areas of expansion in large scale. To explain such expansion one needs to observe what cause the expansion, what forces create such expansion.
On the other hand we do have contractions, and again what forces create such attraction.
Seeking the answers through science investigation is the key.
CMB research the results and find the answers. Search CMB in arXiv

Submitted on 16 Jul 2021]
Four-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: On-sky Receiver Performance at 40, 90, 150, and 220 GHz Frequency Bands
Sumit Dahal, John W. Appel, Rahul Datta, Michael K. Brewer, Aamir Ali, Charles L. Bennett, Ricardo Bustos, Manwei Chan, David T. Chuss, Joseph Cleary, Jullianna D. Couto, Kevin L. Denis, Rolando Dünner, Joseph Eimer, Francisco Espinoza, Thomas Essinger-Hileman, Joseph E. Golec, Kathleen Harrington, Kyle Helson, Jeffrey Iuliano, John Karakla, Yunyang Li, Tobias A. Marriage, Jeffrey J. McMahon, Nathan J. Miller, Sasha Novack, Carolina Núñez, Keisuke Osumi, Ivan L. Padilla, Gonzalo A. Palma, Lucas Parker, Matthew A. Petroff, Rodrigo Reeves, Gary Rhoades, Karwan Rostem, Deniz A. N. Valle, Duncan J. Watts, Janet L. Weiland, Edward J. Wollack, Zhilei Xu
The Cosmology Large Angular Scale Surveyor (CLASS) observes the polarized cosmic microwave background (CMB) over the angular scales of 1∘≲θ≤ 90∘ with the aim of characterizing primordial gravitational waves and cosmic reionization. We report on the on-sky performance of the CLASS Q-band (40 GHz), W-band (90 GHz), and dichroic G-band (150/220 GHz) receivers that have been operational at the CLASS site in the Atacama desert since June 2016, May 2018, and September 2019, respectively. We show that the noise-equivalent power measured by the detectors matches the expected noise model based on on-sky optical loading and lab-measured detector parameters. Using Moon, Venus, and Jupiter observations, we obtain power-to-antenna-temperature calibrations and optical efficiencies for the telescopes. From the CMB survey data, we compute instantaneous array noise-equivalent-temperature sensitivities of 22, 19, 24, and 56 μKcmbs√ for the 40, 90, 150, and 220 GHz frequency bands, respectively. These noise temperatures refer to white noise amplitudes, which contribute to sky maps at all angular scales. Future papers will assess additional noise sources impacting larger angular scales.
Comments:13 pages, 3 figures, submitted to ApJ
Subjects:Instrumentation and Methods for Astrophysics (astro-ph.IM); Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Cite as:arXiv:2107.08022 [astro-ph.IM]
(or arXiv:2107.08022v1 [astro-ph.IM] for this version)
CMB is the faint radiation that abounds through out the universe.
You would expect the CMB to be so and not from a start.
Years gone by people were looking for a creation model that the churches would approve of and than the politicians would hold onto. Scientists were funded if they showed attention and wrote papers along those lines. Schools were educated along the BBT and so the theory became dominant.
Scientists that did not, they would not be funded.
Feb 3, 2020
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Gravity travelling at the speed of light? That is an extraordinary concept. Radiation, like visible light, seems to be the result of dynamic processes such as those that might occur within a star. Gravity, as I understand it, is likely caused by mass. Mass doesn't seem overly dynamic. I wonder what energy creates its waves?
If you assume the BBT is correct than what you see as CMB is evidence.
Is that right or is it wrong.

Gravity as in Gauge field theoretically travels at the speed of light.

Temp of the CMB should be constant indicating the existence of the universe as infinite.
We cannot create more matter we cannot destroy matter, thus what is will always be.
So the CMB must be constant.