The problem I have with that is that if the universe is infinite, then it is infinite - not limited to 13.8 billion years + expansion. If the universe is expanding, then one would assume that the part that is beyond visible limits is also expanding. I suspect I'm making an incorrect assumption here, because I think there is a reason to consider the universe as infinite - but, I don't know that reason. Physicists can't detect anything beyond the visible limit as the limit is defined by the speed of light - so nothing is fast enough to get here even after infinite time. So whatever is beyond the visible limit is lost to us. As of now, physicists say the univers is continuing to expand. In fact, it is expanding at an increasingly high rate. Physicists actually found a force that is causing this increased expansion rate. Thus, the end of this universe is believed to be a total evaporation, with all energy (including all mass) spreading to nothingness in absolute dark and cold.
We haven't identified the force itself. We only know it exists because we have measured an accelerating expansion. And we give it the name, Dark Energy. No one has ever detected Dark Energy in a lab. But we can observe the effects at very large scale.
Thanks. I appreciate hearing the precise case. I will say that detecting dark energy in a lab seems quite unlikely for the foreseeable future given its characteristics.
And as far as I know, there is no model that predicts it. We observe the effects and that's all we know. There is a lot of confusion about dark energy and dark matter, and understandably so.
Clearly. I think I identified this as current theory. There could be significant findings supporting some new model.
Theories of dark energy[edit] Dark energy's status as a hypothetical force with unknown properties makes it a very active target of research. The problem is attacked from a great variety of angles, such as modifying the prevailing theory of gravity (general relativity), attempting to pin down the properties of dark energy, and finding alternative ways to explain the observational data. The equation of state of Dark Energy for 4 common models by Redshift.[43] A: CPL Model, B: Jassal Model, C: Barboza & Alcaniz Model, D: Wetterich Model Cosmological constant[edit] Main article: Cosmological constant Further information: Equation of state (cosmology) Estimated distribution of matter and energy in the universe[44] The simplest explanation for dark energy is that it is an intrinsic, fundamental energy of space. This is the cosmological constant, usually represented by the Greek letter Λ (Lambda, hence Lambda-CDM model). Since energy and mass are related according to the equation E = mc2, Einstein's theory of general relativity predicts that this energy will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty vacuum. The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate. The reason a cosmological constant has negative pressure can be seen from classical thermodynamics. In general, energy must be lost from inside a container (the container must do work on its environment) in order for the volume to increase. Specifically, a change in volume dV requires work done equal to a change of energy −P dV, where P is the pressure. But the amount of energy in a container full of vacuum actually increases when the volume increases, because the energy is equal to ρV, where ρ is the energy density of the cosmological constant. Therefore, P is negative and, in fact, P = −ρ. There are two major advantages for the cosmological constant. The first is that it is simple. Einstein had in fact introduced this term in his original formulation of general relativity such as to get a static universe. Although he later discarded the term after Hubble found that the universe is expanding, a nonzero cosmological constant can act as dark energy, without otherwise changing the Einstein field equations. The other advantage is that there is a natural explanation for its origin. Most quantum field theories predict vacuum fluctuations that would give the vacuum this sort of energy. This is related to the Casimir effect, in which there is a small suction into regions where virtual particles are geometrically inhibited from forming (e.g. between plates with tiny separation). A major outstanding problem is that the same quantum field theories predict a huge cosmological constant, more than 100 orders of magnitude too large.[11] This would need to be almost, but not exactly, cancelled by an equally large term of the opposite sign. Some supersymmetric theories require a cosmological constant that is exactly zero,[45] which does not help because supersymmetry must be broken. Nonetheless, the cosmological constant is the most economical solution to the problem of cosmic acceleration. Thus, the current standard model of cosmology, the Lambda-CDM model, includes the cosmological constant as an essential feature. Quintessence[edit] Main article: Quintessence (physics) In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical field, referred to as quintessence field. Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, the field must be very light so that it has a large Compton wavelength. No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein's equivalence principle and variation of the fundamental constants in space or time.[46] Scalar fields are predicted by the Standard Model of particle physics and string theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmological inflation) occurs: renormalization theory predicts that scalar fields should acquire large masses. The coincidence problem asks why the acceleration of the Universe began when it did. If acceleration began earlier in the universe, structures such as galaxies would never have had time to form, and life, at least as we know it, would never have had a chance to exist. Proponents of the anthropic principle view this as support for their arguments. However, many models of quintessence have a so-called "tracker" behavior, which solves this problem. In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter-radiation equality, which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the low energy scale of the dark energy.[47][48] In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w = −1) from above to below. A No-Go theorem has been proved that gives this scenario at least two degrees of freedom as required for dark energy models. This scenario is so-called Quintom scenario. Some special cases of quintessence are phantom energy, in which the energy density of quintessence actually increases with time, and k-essence (short for kinetic quintessence) which has a non-standard form of kinetic energy such as a negative kinetic energy.[49] They can have unusual properties: phantom energy, for example, can cause a Big Rip. Interacting dark energy[edit] This class of theories attempts to come up with an all-encompassing theory of both dark matter and dark energy as a single phenomenon that modifies the laws of gravity at various scales. This could, for example, treat dark energy and dark matter as different facets of the same unknown substance,[50] or postulate that cold dark matter decays into dark energy.[51] Another class of theories that unifies dark matter and dark energy are suggested to be covariant theories of modified gravities. These theories alter the dynamics of the space-time such that the modified dynamic stems what have been assigned to the presence of dark energy and dark matter.[52] Variable dark energy models[edit] The density of dark energy might have varied in time over the history of the universe. Modern observational data allow for estimates of the present density. Using baryon acoustic oscillations, it is possible to investigate the effect of dark energy in the history of the Universe, and constrain parameters of the equation of state of dark energy. To that end, several models have been proposed. One of the most popular models is the Chevallier–Polarski–Linder model (CPL).[53][54] Some other common models are, (Barboza & Alcaniz. 200,[55] (Jassal et al. 2005),[56] (Wetterich. 2004),[57](Oztas et al. 201.[58][59 https://en.wikipedia.org/wiki/Dark_energy#Theories_of_dark_energy
Dark matter Composition of dark matter: baryonic vs. nonbaryonic[edit] There are various hypotheses about what dark matter could consist of, as set out in the table below. Light bosons quantum chromodynamics axions axion-like particles fuzzy cold dark matter neutrinos Standard Model sterile neutrinos weak scale supersymmetry extra dimensions little Higgs effective field theory simplified models other particles WIMPzilla self-interacting dark matter superfluid vacuum theory macroscopic primordial black holes massive compact halo objects (MaCHOs) Macroscopic dark matter (Macros) modified gravity (MOG) modified Newtonian dynamics (MoND) Tensor–vector–scalar gravity (TeVeS) Entropic gravity Dark matter can refer to any substance that interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons.[note 4] However, for the reasons outlined below, most scientists think the dark matter is dominated by a non-baryonic component, which is likely composed of a currently unknown fundamental particle (or similar exotic state). ">Play mediaFermi-LAT observations of dwarf galaxies provide new insights on dark matter. Baryonic matter[edit] Not to be confused with Missing baryon problem. Baryons (protons and neutrons) make up ordinary stars and planets. However, baryonic matter also encompasses less common black holes, neutron stars, faint old white dwarfs and brown dwarfs, collectively known as massive compact halo objects (MACHOs), which can be hard to detect.[84] However, multiple lines of evidence suggest the majority of dark matter is not made of baryons: Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars. The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.[85][86] Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.[77] Astronomical searches for gravitational microlensing in the Milky Way found that at most a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30 solar masses, which covers nearly all the plausible candidates.[87][88][89][90][91][92] Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background.[93] Observations by WMAP and Planck indicate that around five sixths of the total matter is in a form that interacts significantly with ordinary matter or photons only through gravitational effects. Non-baryonic matter[edit] Candidates for non-baryonic dark matter are hypothetical particles such as axions, sterile neutrinos, weakly interacting massive particles (WIMPs), gravitationally-interacting massive particles (GIMPs), or supersymmetric particles. The three neutrino types already observed are indeed abundant, and dark, and matter, but because their individual masses—however uncertain they may be—are almost certainly tiny, they can only supply a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.[94] Unlike baryonic matter, nonbaryonic matter did not contribute to the formation of the elements in the early universe (Big Bang nucleosynthesis)[13] and so its presence is revealed only via its gravitational effects, or weak lensing. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves, possibly resulting in observable by-products such as gamma rays and neutrinos (indirect detection).[94] Dark matter aggregation and dense dark matter objects[edit] If dark matter is as common as observations suggest, an obvious question is whether it can form objects equivalent to planets, stars, or black holes. The answer has historically been that it cannot,[95][96] because of two factors: It lacks an efficient means to lose energy:[95] Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase velocity and momentum. Dark matter appears to lack means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The Virial theorem suggests that such a particle would not stay bound to the gradually forming object—as the object began to form and compact, the dark matter particles within it would speed up and tend to escape. It lacks a range of interactions needed to form structures:[96] Ordinary matter interacts in many different ways. This allow it to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of neutrinos and electromagnetic radiation through fusion when they become energetic enough. Protons and neutrons can bind via the strong interaction and then form atoms with electrons largely through electromagnetic interaction. But there is no evidence that dark matter is capable of such a wide variety of interactions, since it only seems to interact through gravity and through some means no stronger than the weak interaction (although this is speculative until dark matter is better understood). In 2015–2017 the idea that dense dark matter was composed of primordial black holes, made a comeback[97] following results of gravitation wave measurements which detected the merger of intermediate mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses). It was proposed that the intermediate mass black holes causing the detected merger formed in the hot dense early phase of the universe due to denser regions collapsing. However this was later ruled out by a survey of about a thousand supernova which detected no gravitational lensing events, although about 8 would be expected if intermediate mass primordial black holes accounted for the majority of dark matter.[98] The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the suns heliosphere by the Voyager 1 spacecraft. Tiny black holes are theorized to emit Hawking radiation. However the detected fluxes were too low and did not have the expected energy spectrum suggesting that tiny primordial black holes are not widespread enough to account for dark matter.[99] None-the-less research and theories proposing that dense dark matter account for dark matter continue as of 2018, including approaches to dark matter cooling,[100][101] and the question remains unsettled.
Classification of dark matter: cold, warm or hot[edit] Dark matter can be divided into cold, warm, and hot categories.[102] These categories refer to velocity rather than an actual temperature, indicating how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion—this is an important distance called the free streaming length (FSL). Primordial density fluctuations smaller than this length get washed out as particles spread from overdense to underdense regions, while larger fluctuations are unaffected; therefore this length sets a minimum scale for later structure formation. The categories are set with respect to the size of a protogalaxy (an object that later evolves into a dwarf galaxy): dark matter particles are classified as cold, warm, or hot according to their FSL; much smaller (cold), similar to (warm), or much larger (hot) than a protogalaxy.[103][104] Mixtures of the above are also possible: a theory of mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of dark energy.[citation needed] Cold dark matter leads to a bottom-up formation of structure with galaxies forming first and galaxy clusters at a latter stage, while hot dark matter would result in a top-down formation scenario with large matter aggregations forming early, later fragmenting into separate galaxies;[clarification needed] the latter is excluded by high-redshift galaxy observations.[14] Alternative definitions[edit] These categories also correspond to fluctuation spectrum effects and the interval following the Big Bang at which each type became non-relativistic. Davis et al. wrote in 1985:[105] Candidate particles can be grouped into three categories on the basis of their effect on the fluctuation spectrum (Bond et al. 1983). If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino ... A second possibility is for the dark matter particles to interact more weakly than neutrinos, to be less abundant, and to have a mass of order 1 keV. Such particles are termed "warm dark matter", because they have lower thermal velocities than massive neutrinos ... there are at present few candidate particles which fit this description. Gravitinos and photinos have been suggested (Pagels and Primack 1982; Bond, Szalay and Turner 1982) ... Any particles which became nonrelativistic very early, and so were able to diffuse a negligible distance, are termed "cold" dark matter (CDM). There are many candidates for CDM including supersymmetric particles. — M. Davis, G. Efstathiou, C. S. Frenk, and S. D. M. White, The evolution of large-scale structure in a universe dominated by cold dark matter Another approximate dividing line is that warm dark matter became non-relativistic when the universe was approximately 1 year old and 1 millionth of its present size and in the radiation-dominated era (photons and neutrinos), with a photon temperature 2.7 million K. Standard physical cosmology gives the particle horizon size as 2ct (speed of light multiplied by time) in the radiation-dominated era, thus 2 light-years. A region of this size would expand to 2 million light-years today (absent structure formation). The actual FSL is approximately 5 times the above length, since it continues to grow slowly as particle velocities decrease inversely with the scale factor after they become non-relativistic. In this example the FSL would correspond to 10 million light-years, or 3 megaparsecs, today, around the size containing an average large galaxy. The 2.7 million K photon temperature gives a typical photon energy of 250 electron-volts, thereby setting a typical mass scale for warm dark matter: particles much more massive than this, such as GeV–TeV mass WIMPs, would become non-relativistic much earlier than one year after the Big Bang and thus have FSLs much smaller than a protogalaxy, making them cold. Conversely, much lighter particles, such as neutrinos with masses of only a few eV, have FSLs much larger than a protogalaxy, thus qualifying them as hot. Cold dark matter[edit] Main article: Cold dark matter Cold dark matter offers the simplest explanation for most cosmological observations. It is dark matter composed of constituents with an FSL much smaller than a protogalaxy. This is the focus for dark matter research, as hot dark matter does not seem capable of supporting galaxy or galaxy cluster formation, and most particle candidates slowed early. The constituents of cold dark matter are unknown. Possibilities range from large objects like MACHOs (such as black holes[106] and Preon stars[107]) or RAMBOs (such as clusters of brown dwarfs), to new particles such as WIMPs and axions. Studies of Big Bang nucleosynthesis and gravitational lensing convinced most cosmologists[14][108][109][110][111][112] that MACHOs[108][110] cannot make up more than a small fraction of dark matter.[13][108] According to A. Peter: "... the only really plausible dark-matter candidates are new particles."[109] The 1997 DAMA/NaI experiment and its successor DAMA/LIBRA in 2013, claimed to directly detect dark matter particles passing through the Earth, but many researchers remain skeptical, as negative results from similar experiments seem incompatible with the DAMA results. Many supersymmetric models offer dark matter candidates in the form of the WIMPy Lightest Supersymmetric Particle (LSP).[113] Separately, heavy sterile neutrinos exist in non-supersymmetric extensions to the standard model that explain the small neutrino mass through the seesaw mechanism. Warm dark matter[edit] Main article: Warm dark matter Warm dark matter comprises particles with an FSL comparable to the size of a protogalaxy. Predictions based on warm dark matter are similar to those for cold dark matter on large scales, but with less small-scale density perturbations. This reduces the predicted abundance of dwarf galaxies and may lead to lower density of dark matter in the central parts of large galaxies. Some researchers consider this a better fit to observations. A challenge for this model is the lack of particle candidates with the required mass ~ 300 eV to 3000 eV.[citation needed] No known particles can be categorized as warm dark matter. A postulated candidate is the sterile neutrino: a heavier, slower form of neutrino that does not interact through the weak force, unlike other neutrinos. Some modified gravity theories, such as scalar–tensor–vector gravity, require "warm" dark matter to make their equations work. Hot dark matter[edit] Main article: Hot dark matter Hot dark matter consists of particles whose FSL is much larger than the size of a protogalaxy. The neutrino qualifies as such particle. They were discovered independently, long before the hunt for dark matter: they were postulated in 1930, and detected in 1956. Neutrinos' mass is less than 10−6 that of an electron. Neutrinos interact with normal matter only via gravity and the weak force, making them difficult to detect (the weak force only works over a small distance, thus a neutrino triggers a weak force event only if it hits a nucleus head-on). This makes them 'weakly interacting light particles' (WILPs), as opposed to WIMPs. The three known flavours of neutrinos are the electron, muon, and tau. Their masses are slightly different. Neutrinos oscillate among the flavours as they move. It is hard to determine an exact upper bound on the collective average mass of the three neutrinos (or for any of the three individually). For example, if the average neutrino mass were over 50 eV/c2 (less than 10−5 of the mass of an electron), the universe would collapse. CMB data and other methods indicate that their average mass probably does not exceed 0.3 eV/c2. Thus, observed neutrinos cannot explain dark matter.[114] Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies that the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies. Deep-field observations show instead that galaxies formed first, followed by clusters and superclusters as galaxies clump together. https://en.wikipedia.org/wiki/Dark_matter#Composition_of_dark_matter:_baryonic_vs._nonbaryonic
The hypothesis of the end of the universe via "total evaporation" is not the only one. The Cyclical Universe Model postulates that expansion stops and then cooling begins which contracts matter back into another Singularity which in turn gives rise to the next BB. My own personal version of the Cyclical model is that the universe is infinitely large and has something that I call "Space Weather". Like weather here on Earth is caused by regional temperature differences Space Weather could have similar temperature differences. We are currently in a Warm High region and so we are expanding into Cooler Low regions around us. At some point we will become cooler than the surrounding regions and then the process is reversed. My version allows for there to be multiple BB's occurring in remote regions that we perceive as Super Novas. Yes, 100% speculation on my part but we still have a very long way to go in order to learn all there is to know.
Humans is one species. Dinosaurs comprise of 1000's of species. Viruses are almost 100% species dependent. Cross species viruses never wipe out species. When humans catch Myxomatosis you might have a point
"I thought that cosmic background radiation existed in all areas of the known Universe". It does. Effectively it is just the average temperature of the universe. Bit like an oven that is slowly cooling by increasing it's internal volume.
I suppose infinite might be defined as expansion can continue to infinity? However, if we think our Universe started with the BB, and if we can calculate this happening about 13.8 billion years ago, then we calculate the total expansion since the BB, this can tell us where the current boundary of the Universe should be. But we don't know the rate of expansion beyond the visible Universe so who knows what is happening 'out there'? Is there a good reason to consider the Universe is finite? Perhaps there are limits how far expansion can go before stuff begins to decay?
Interesting! I hadn't heard that version of the "end" of the universe. It does seem attractive. It would be interesting to know what that means concerning the observation that expansion is accelerating. Something would need to overcome that acceleration. That is, at some time in the unimaginably distant future the universe would be collapsing back to our size. But, when it reaches the point at where we are today, the forces would seem to be oriented to accelerating expansion, not allowing further collapse. Cool. It's space that is expanding, right? I haven't seen the idea of the expansion of space being at a different rate in different places. I don't know why not, though (meaningless as I'm not a physicist). If the BB were regional, it would at least have to be farther away than our observable limits, I think. After all, physicists today trace the origin of the observable galaxies to our own BB. It might be interesting to see if anything is arriving from beyond that horizon - something coming from some other BB, to far away to see. So far, I think we're only seeing matter leaving our visual range, but ... I like the idea of multiple BB's. Just cruising into a slowly cooling oblivion is vaguely disappointing. It would be great if there were more astrophysicists hanging around this board!
Yes, I don't know how to think about the size of the universe and infinity. One problem is that I think distance is a characteristic of our universe - like time is. And, it is space itself that is expanding. I don't believe we can talk about the size of the universe as if we are outside the universe. My understanding is that there is pretty much just one rate of expansion, but that the expansion rate has changed over time. Initially, it was unimaginably massive. Then, it slowed. Today, they say the expansion rate is increasing. I think the most popular theory of the end of the universe is that it will continue to expand and thus cool. Mass will decay, leading to a universe filled by an energy field that is gradually dissipating.
Since the discovery of dark energy, I think all hypotheses in this respect have to be reconsidered. In fact, until we get a handle on the nature of dark energy, I'd say all bets are off. As far as we know, the universe will not only continue to expand, but continue to accelerate forever.
There are actually several different versions of the Cyclical Model including this one embraced by Roger Penrose. https://physicsworld.com/a/new-evidence-for-cyclic-universe-claimed-by-roger-penrose-and-colleagues/
Wow - blowing away inflation like that would be huge for the world of cosmology! And, Penrose has the credentials. Thanks!
'Outside the Universe' is interesting. Watching the Universe expand we would see what it is expanding into? If the expansion has a place to go what are the properties of 'that place'? We would not know the limits to expansion so we won't know if it goes forever or stops? We could check it out every billion years or so to see how it unfolds. Why do we need to theorize that there will be an end to the Universe? The time and distances already are difficult to fathom so why can't this be infinite? We believe the Sun will die out in ~5 billion years so solar systems are doomed. Galaxies can be wiped out. So it appears lots of the Universe is just temporary...so maybe there is a mechanism that eventually renders the Universe null and void? That would be a shame! But...will all of the light and dark matter and energy vaporize into nothingness or is it redistributed waiting to again coalesce into galaxies and solar systems?
One of the interesting aspects is that today's physics doesn't answer everything. That is, we know that there exists a better understanding. And, these questions come from the small of particle physics to the large of cosmology and includes issues such as gravity. I don't believe we can guess at the monetary value of having a better understanding of how our universe works.
Firstly, you wouldn't see anything. There is no "place" - having a place means having space, meaning you would be in this universe. Seeing the universe requires the light to reach you. And it would never reach you unless you are in this universe. It is natural for us to try to step back and view things from what we used to call the God Frame of Reference, at my university. It is natural when doing problems in Relativity. But the God Frame doesn't exist. Physics doesn't ask why. It asks, what? There is no "need" for anything. We go where the evidence and math takes us.
A classic example of the nonexistent God Frame often comes up when students first learn about relative motion. For example, consider an object from deep space passing the earth [using the sun as a reference point] at a speed of 40,000 mph. We launch a probe which eventually matches the speed of the object. If you were on the probe, because you are not experiencing any accelerations [no thrusters running], you would consider yourself, and measure the object from deep space to be at rest relative to your frame of reference. You would measure the earth/sun to receding at a speed of 40,000 mph. So you can define that the earth is in motion, and not you. Likewise, on earth we define the sun to be at rest, and the object from deep space to be in motion at 40,000 mph. So who is right? In other words, how would things look if you could step outside of normal space and see who is REALLY moving. What would God say? And the answer is that both observers are right. There is no absolute answer otherwise. That is the meaning of [constant] motion being relative. It all comes down to the idea that we can somehow step out of space and time, and see the universe as it really is. But that frame of reference doesn't exist. Reality itself [length, mass, time] is determined by your state of motion relative to everything else.
