Astronomy/Black Holes & Big Bang
Presumably Black Holes / Singularities give birth to new universes. Our universe is expected to expand to a Heat Death rather than collapse. The largest Black Holes detected may be millions (?) of solar masses, but infinitesimal compared to the mass of the Universe. So how did a Universe-amount of matter get into the Singularity that became our Universe? Or is all the matter in a Black Hole left in its native Universe, and the Singularity creates matter anew in the new Universe?
I sure wish I could answer your question definitively. But I'm in good company. Because no one can.
Please let me break down your question into parts, and I'll address each part:
"Presumably Black Holes / Singularities give birth to new universes".
I believe you're referring to Stephen Hawking's series of essays on some speculative science. Although Hawking is certainly a very respected theoretical physicist, he has also developed some untestable theories regarding black hole thermodynamics, and the possible link between black holes and "baby universes". A lesser known, but also very brilliant physicist, is Prof. Andrei Linde of Stanford - see http://www.stanford.edu/~alinde/
. He attempts to explain the apparent accelerated expansion of the universe and inflation of the early universe in the context of multiple universes and string theory.
These ideas are very interesting and make fascinating reading, but it could be all very wrong. Or not. We just don't know! And we probably will NEVER know. Physics is based on developing theories (models) which can be tested by experiment or observation. The model makes certain predictions, which are then tested. If the predictions are close to observations, then it's a good model. If it isn't, then there's a problem with the model and a new one is sought.
Many cosmological models are not testable. They're not open to observation. And there's active debate among some experts of their basic principles. Is the universe expanding? The majority of physicists believe so, but not all! Is it accelerating? Even more debate! And what about string theory? There's considerable debate, since currently, string theory does not make testable predictions.
I urge you to keep an open mind on these questions. Science never provides definitive "answers" - just models that work under a given set of parameters.
"Our universe is expected to expand to a Heat Death rather than collapse."
When you say "expected", please realize that this is only an (unproven) idea. It is 'generally' assumed that the Big Bang occurred some 13.7 billion years ago, and the universe will continue to expand (even accelerate) until the energy density in the universe becomes extremely low, resulting in a "Heat Death". Although this idea is believed to be valid by the majority of cosmologists, it is by no means universally accepted. The "Big Bang", based on Hubble's velocity-distance relation and the microwave background radiation, is very much still a theory, and there are alternative theories. Some observations do not support an expanding universe or the Big Bang. There are several references concerning this, but perhaps the most authoritative person is Halton Arp, a leading astronomer and researcher on galaxies, who wrote "Seeing Red". That book is highly recommended to get an alternative view. Or read "A Different Approach to Cosmology" by Hoyle, Burbidge, and Narlikar. That's another great book which gives a scientific view on how the universe has always been in a steady state.
"The largest Black Holes detected may be millions (?) of solar masses, but infinitesimal compared to the mass of the Universe."
Yes, it is generally assumed that very large black holes exist in the centers of most galaxies. And we've seen evidence of such black holes through intense high-energy radiation which seems to be spiraling into them.
"So how did a Universe-amount of matter get into the Singularity that became our Universe? Or is all the matter in a Black Hole left in its native Universe, and the Singularity creates matter anew in the new Universe?"
This is where we're really getting into untestable speculation. Whether a singularity existed in the early universe (or in black holes) is unknown. Current theory says that no known matter can resist the gravitational force that exists in black holes (or the very dense early universe). But we don't know the effect of quantum fluctuations at that density. So we could be left with a dense quantum foam rather than a singularity. We just don't know.
This speculation reminds me of string theory. A very good book on string theory and its inability to prove anything is "Not even Wrong" by Peter Woit. In it, he argues that any scientific theory should be testable and predict certain results, which string theory can't. It can't be proven right or wrong. It's simply a way of trying to make sense of the universe, but is not a testable scientific theory. See http://www.math.columbia.edu/~woit/
So unless a theory is tested by the scientific method (i.e., predicts certain outcomes and reproduces predicted results), it must be taken with a grain of salt.
Prof. James Gort